Guidance for Industry
Statistical Approaches to
Establishing Bioequivalence
U.S. Department of Health and Human Services
Food and Drug Administration
Center for Drug Evaluation and Research (CDER)
January 2001
BP Guidance for Industry
Statistical Approaches to
Establishing Bioequivalence
Additional copies are available from:
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(Internet)  http://www.fda.gov/cder/guidance/index.htm
U.S. Department of Health and Human Services
Food and Drug Administration
Center for Drug Evaluation and Research (CDER)
January 2001
BP Table of Contents
I. INTRODUCTION..................................................................................................................................................................1
II. BACKGROUND....................................................................................................................................................................1
A. GENERAL.............................................................................................................................................................................1
B. STATISTICAL......................................................................................................................................................................2
III. STATISTICAL MODEL......................................................................................................................................................3
IV. STATISTICAL APPROACHES FOR BIOEQUIVALENCE.........................................................................................3
A. AVERAGE BIOEQUIVALENCE..............................................................................................................................................4
B. POPULATION BIOEQUIVALENCE.........................................................................................................................................5
C. INDIVIDUAL BIOEQUIVALENCE...........................................................................................................................................6
V. STUDY DESIGN....................................................................................................................................................................7
A. EXPERIMENTAL DESIGN.....................................................................................................................................................7
B. SAMPLE SIZE AND DROPOUTS...........................................................................................................................................8
VI. STATISTICAL ANALYSIS................................................................................................................................................9
A. LOGARITHMIC TRANSFORMATION....................................................................................................................................9
B. DATA ANALYSIS..............................................................................................................................................................10
VII. MISCELLANEOUS ISSUES........................................................................................................................................13
A. STUDIES IN MULTIPLE GROUPS.......................................................................................................................................13
B. CARRYOVER EFFECTS......................................................................................................................................................13
C. OUTLIER CONSIDERATIONS.............................................................................................................................................14
D. DISCONTINUITY................................................................................................................................................................15
REFERENCES................................................................................................................................................................................16
APPENDIX A..................................................................................................................................................................................21
APPENDIX B..................................................................................................................................................................................25
APPENDIX C..................................................................................................................................................................................28
APPENDIX D..................................................................................................................................................................................32
APPENDIX E..................................................................................................................................................................................34
APPENDIX F..................................................................................................................................................................................35
APPENDIX G..................................................................................................................................................................................40
APPENDIX H..................................................................................................................................................................................45 1
GUIDANCE FOR INDUSTRY
Statistical Approaches
to Establishing Bioequivalence
This guidance represents the Food and Drug Administration's current thinking on this topic.  It
does not create or confer any rights for or on any person and does not operate to bind FDA or the
public.  An alternative approach may be used if such approach satisfies the requirements of the
applicable statutes and regulations.
I. INTRODUCTION
This guidance provides recommendations to sponsors and applicants who intend, either before or after
approval, to use equivalence criteria in analyzing in vivo or in vitro bioequivalence (BE) studies for
investigational new drug applications (INDs), new drug applications (NDAs), abbreviated new drug
applications (ANDAs) and supplements to these applications.  This guidance discusses three
approaches for BE comparisons:  average, population, and individual. The guidance focuses on how to
use each approach once a specific approach has been chosen.  This guidance replaces a prior FDA
guidance entitled Statistical Procedures for Bioequivalence Studies Using a Standard Two-
Treatment Crossover Design, which was issued in July 1992.
II. BACKGROUND
A. General
Requirements for submitting bioavailability (BA) and BE data in NDAs, ANDAs, and
supplements, the definitions of BA and BE, and the types of in vivo studies that are appropriate
to measure BA and establish BE are set forth in 21 CFR part 320.  This guidance provides
recommendations on how to meet provisions of part 320 for all drug products.
Defined as relative BA, BE involves comparison between a test (T) and reference (R) drug
product, where T and R can vary, depending on the comparison to be performed (e.g., to-be-
marketed dosage form versus clinical trial material, generic drug versus reference listed drug,
                    
1 This guidance has been prepared by the Population and Individual Bioequivalence Working Group of the
Biopharmaceutics Coordinating Committee in the Office of Pharmaceutical Science, Center for Drug Evaluation and
Research (CDER) at the Food and Drug Administration (FDA).
J:\!GUIDANC\3616fnl.doc
01/31/01 drug product changed after approval versus drug product before the change).  Although BA
and BE are closely related, BE comparisons normally rely on (1) a criterion, (2) a confidence
interval for the criterion, and (3) a predetermined BE limit.  BE comparisons could also be used
in certain pharmaceutical product line extensions, such as additional strengths, new dosage
forms (e.g., changes from immediate release to extended release), and new routes of
administration.  In these settings, the approaches described in this guidance can be used to
determine BE.  The general approaches discussed in this guidance may also be useful when
assessing pharmaceutical equivalence or performing equivalence comparisons in clinical
pharmacology studies and other areas.
B. Statistical
In the July 1992 guidance on Statistical Procedures for Bioequivalence Studies Using a
Standard Two-Treatment Crossover Design (the 1992 guidance), CDER recommended that
a standard in vivo BE study design be based on the administration of either single or multiple
doses of the T and R products to healthy subjects on separate occasions, with random
assignment to the two possible sequences of drug product administration.  The 1992 guidance
further recommended that statistical analysis for pharmacokinetic measures, such as area under
the curve (AUC) and peak concentration (Cmax), be based on the two one-sided tests
procedure to determine whether the average values for the pharmacokinetic measures
determined after administration of the T and R products were comparable.  This approach is
termed average bioequivalence and involves the calculation of a 90% confidence interval for
the ratio of the averages (population geometric means) of the measures for the T and R
products.  To establish BE, the calculated confidence interval should fall within a BE limit,
usually 80-125% for the ratio of the product averages.2  In addition to this general approach,
the 1992 guidance provided specific recommendations for (1) logarithmic transformation of
pharmacokinetic data, (2) methods to evaluate sequence effects, and (3) methods to evaluate
outlier data.
Although average BE is recommended for a comparison of BA measures in most BE studies,
this guidance describes two new approaches, termed population and individual
bioequivalence.  These new approaches may be useful, in some instances, for analyzing
in vitro and in vivo BE studies.3  The average BE approach focuses only on the comparison of
population averages of a BE measure of interest and not on the variances of the measure for the
                    
2
 For a broad range of drugs, a BE limit of 80 to 125% for the ratio of the product averages has been adopted
for use of an average BE criterion.  Generally, the BE limit of 80 to 125% is based on a clinical judgment that a test
product with BA measures outside this range should be denied market access. 
3
  For additional recommendations on in vivo studies, see the FDA guidance for industry on Bioavailability
and Bioequivalence Studies for Orally Administered Drug Products C General Considerations.  Additional
recommendations on in vitro studies will be provided in an FDA guidance for industry on Bioavailability and
Bioequivalence Studies for Nasal Aerosols and Nasal Sprays for Local Action, when finalized.
2 T and R products.  The average BE method does not assess a subject-by-formulation
interaction variance, that is, the variation in the average T and R difference among individuals. 
In contrast, population and individual BE approaches include comparisons of both averages and
variances of the measure.  The population BE approach assesses total variability of the measure
in the population.  The individual BE approach assesses within-subject variability for the T and
R products, as well as the subject-by-formulation interaction.
III. STATISTICAL MODEL
Statistical analyses of BE data are typically based on a statistical model for the logarithm of the BA
measures (e.g., AUC and Cmax).  The model is a mixed-effects or two-stage linear model. Each
subject, j, theoretically provides a mean for the log-transformed BA measure for each formulation, m
Tj
and m for the T and R formulations, respectively.  The model assumes that these subject-specific
Rj 
means come from a distribution with population means m  and m , and between-subject variances s 2
T R BT
and s 2, respectively.  The model allows for a correlation, r , between m  and m .  The subject-by-
BR Tj Rj
formulation interaction variance component (Schall and Luus 1993), s 2, is related to these parameters
D
as follows:
s 2  = variance of (m - m )
D Tj  Rj
       = (s  - s )2 + 2 (1-r )s s Equation 1
BT BR BT BR
For a given subject, the observed data for the log-transformed BA measure are assumed to be
independent observations from distributions with means m  and m , and within-subject variances s 2
Tj Rj WT
and s 2.  The total variances for each formulation are defined as the sum of the within- and between-
WR
subject components (i.e., s 2 = s 2 + s 2 and s 2 = s 2 + s 2).  For analysis of crossover
TT WT BT TR WR BR
studies, the means are given additional structure by the inclusion of period and sequence effect terms.
IV. STATISTICAL APPROACHES FOR BIOEQUIVALENCE
The general structure of a BE criterion is that a function (Q ) of population measures should be
demonstrated to be no greater than a specified value (q ).  Using the terminology of statistical hypothesis
testing, this is accomplished by testing the hypothesis H: Q >q  versus H : Q #q  at a desired level of
0 A
significance, often 5%.  Rejection of the null hypothesis H (i.e., demonstrating that the estimate of Q  is
0
statistically significantly less than )q  results in a conclusion of BE.  The choice of Q  and q  differs in
average, population, and individual BE approaches.
A general objective in assessing BE is to compare the log-transformed BA measure after administration
of the T and R products.  As detailed in Appendix A, population and individual approaches are based
on the comparison of an expected squared distance between the T and R formulations to the expected
3 squared distance between two administrations of the R formulation. An acceptable T formulation is one
where the T-R distance is not substantially greater than the R-R distance.  In both population and
individual BE approaches, this comparison appears as a comparison to the reference variance, which is
referred to as scaling to the reference variability.
Population and individual BE approaches, but not the average BE approach, allow two types of scaling:
 reference-scaling and constant-scaling.  Reference-scaling means that the criterion used is scaled to the
variability of the R product, which effectively widens the BE limit for more variable reference products. 
Although generally sufficient, use of reference-scaling alone could unnecessarily narrow the BE limit for
drugs and/or drug products that have low variability but a wide therapeutic range.  This guidance,
therefore, recommends mixed-scaling for the population and individual BE approaches (section IV.B
and C).  With mixed scaling, the reference-scaled form of the criterion should be used if the reference
product is highly variable; otherwise, the constant-scaled form should be used.
A. Average Bioequivalence
The following criterion is recommended for average BE:
(m  - m )2 # q 2 Equation 2
T R A
where
 m  = population average response of the log-transformed measure for the T
T
         formulation
 m  = population average response of the log-transformed measure for the R
R
         formulation
as defined in section III above.
4 This criterion is equivalent to:
-q  # (m  - m ) # q Equation 3
A T R A
and, usually, q  = ln(1.25).
A
B. Population Bioequivalence
The following mixed-scaling approach is recommended for population BE (i.e., use the
reference-scaled method if the estimate of s  > s  and the constant-scaled method if the
TR T0
estimate of s  # s ). 
TR T0
The recommended criteria are:
!  Reference-Scaled:
 (m  - m )2 + (s 2 - s 2)
T R TT TR
--------------------------------  #  q   Equation 4
p
          s 2
TR
or
! Constant-Scaled:
 (m  - m )2 + (s 2 - s 2)
T R TT TR
--------------------------------  #  q Equation 5
p
           s 2
T0
where:
m   = population average response of the log-transformed measure
T
    for the T formulation
m   = population average response of the log-transformed measure
R
    for the R formulation
s 2 = total variance (i.e., sum of within- and between-subject
TT
    variances) of the T formulation
s 2 = total variance (i.e., sum of within- and between-subject
TR
    variances) of the R formulation
s 2  = specified constant total variance
T0
q = BE limit
p
5 Equations 4 and 5 represent an aggregate approach where a single criterion on the left-hand
side of the equation encompasses two major components:  (1) the difference between the T and
R population averages (m  - m ), and (2) the difference between the T and R total variances
T R
(s 2 - s 2).  This aggregate measure is scaled to the total variance of the R product or to a
TT TR
constant value (s 2, a standard that relates to a limit for the total variance), whichever is
T0
greater.
The specification of both s  and q  relies on the establishment of standards.  The generation of
T0 P
these standards is discussed in Appendix A.  When the population BE approach is used, in
addition to meeting the BE limit based on confidence bounds, the point estimate of the
geometric test/reference mean should fall within 80-125%.
C. Individual Bioequivalence
The following mixed-scaling approach is one approach for individual BE (i.e., use the reference-
scaled method if the estimate of s  > s , and the constant-scaled method if the estimate of
WR W0
s  # s ).  Also see section VII.D, Discontinuity, for further discussion.
WR W0
The recommended criteria are:
!  Reference-Scaled:
 (m  - m )2 + s 2 + (s 2 - s 2)
T R D WT WR
-----------------------------------------  #  q Equation 6
I
       s 2
WR
or
! Constant-Scaled:
 (m  - m )2 + s 2 + (s 2 - s 2)
T R D WT WR
-----------------------------------------  #  q Equation 7
I
              s 2
W0
where:
m   = population average response of the log-transformed measure
T
    for the T formulation
m   = population average response of the log-transformed measure
R
    for the R formulation
s 2 = subject-by-formulation interaction variance component
D
6 s 2 = within-subject variance of the T formulation
WT
s 2 = within-subject variance of the R formulation
WR
s 2  = specified constant within-subject variance
W0
q = BE limit
I
Equations 6 and 7 represent an aggregate approach where a single criterion on the left-hand
side of the equation encompasses three major components:  (1) the difference between the T
and R population averages (m  - m ), (2) subject-by-formulation interaction (s 2), and (3) the
T R D
difference between the T and R within-subject variances (s 2 - s 2).  This aggregate
WT WR
measure is scaled to the within-subject variance of the R product or to a constant value (s 2, a
W0
standard that relates to a limit for the within-subject variance), whichever is greater.
The specification of both s  and q  relies on the establishment of standards.  The generation of
W0 I
these standards is discussed in Appendix A.  When the individual BE approach is used, in
addition to meeting the BE limit based on confidence bounds, the point estimate of the
geometric test/reference mean ratio should fall within 80-125%.
V. STUDY DESIGN
A. Experimental Design
1. Nonreplicated Designs
A conventional nonreplicated design, such as the standard two-formulation, two-period,
two-sequence crossover design, can be used to generate data where an average or
population approach is chosen for BE comparisons.  Under certain circumstances,
parallel designs can also be used.
2. Replicated Crossover Designs
Replicated crossover designs can be used irrespective of which approach is selected to
establish BE, although they are not necessary when an average or population approach
is used.  Replicated crossover designs are critical when an individual BE approach is
used to allow estimation of within-subject variances for the T and R measures and the
subject-by-formulation interaction variance component.  The following four-period,
two-sequence, two-formulation design is recommended for replicated BE studies (see
Appendix B for further discussion of replicated crossover designs).
7 Period
1 2 3 4
1 T R T R
Sequence
2 R T R T
For this design, the same lots of the T and R formulations should be used for the
replicated administration.  Each period should be separated by an adequate washout
period.
Other replicated crossover designs are possible.  For example, a three-period design,
as shown below, could be used.
       Period
1 2 3
1 T R T
Sequence
2 R T R
A greater number of subjects would be encouraged for the three-period design
compared to the recommended four-period design to achieve the same statistical power
to conclude BE (see Appendix C).
B. Sample Size and Dropouts
A minimum number of 12 evaluable subjects should be included in any BE study.  When an
average BE approach is selected using either nonreplicated or replicated designs, methods
appropriate to the study design should be used to estimate sample sizes.  The number of
subjects for BE studies based on either the population or individual BE approach can be
estimated by simulation if analytical approaches for estimation are not available.  Further
information on sample size is provided in Appendix C. 
Sponsors should enter a sufficient number of subjects in the study to allow for dropouts. 
Because replacement of subjects during the study could complicate the statistical model and
analysis, dropouts generally should not be replaced.  Sponsors who wish to replace dropouts
during the study should indicate this intention in the protocol.  The protocol should also state
8 whether samples from replacement subjects, if not used, will be assayed.  If the dropout rate is
high and sponsors wish to add more subjects, a modification of the statistical analysis may be
recommended.  Additional subjects should not be included after data analysis unless the trial
was designed from the beginning as a sequential or group sequential design.
VI. STATISTICAL ANALYSIS
The following sections provide recommendations on statistical methodology for assessment of average,
population, and individual BE.
A. Logarithmic Transformation
1. General Procedures
This guidance recommends that BE measures (e.g., AUC and Cmax) be log-
transformed using either common logarithms to the base 10 or natural logarithms (see
Appendix D).  The choice of common or natural logs should be consistent and should
be stated in the study report.  The limited sample size in a typical BE study precludes a
reliable determination of the distribution of the data set.  Sponsors and/or applicants are
not encouraged to test for normality of error distribution after log-transformation, nor
should they use normality of error distribution as a reason for carrying out the statistical
analysis on the original scale.  Justification should be provided if sponsors or applicants
believe that their BE study data should be statistically analyzed on the original rather
than on the log scale.
2. Presentation of Data
The drug concentration in biological fluid determined at each sampling time point should
be furnished on the original scale for each subject participating in the study.  The
pharmacokinetic measures of systemic exposure should also be furnished on the original
scale.  The mean, standard deviation, and coefficient of variation for each variable
should be computed and tabulated in the final report. 
In addition to the arithmetic mean and associated standard deviation (or coefficient of
variation) for the T and R products, geometric means (antilog of the means of the logs)
should be calculated for selected BE measures.  To facilitate BE comparisons, the
measures for each individual should be displayed in parallel for the formulations tested. 
In particular, for each BE measure the ratio of the individual geometric mean of the T
product to the individual geometric mean of the R product should be tabulated side by
side for each subject.  The summary tables should indicate in which sequence each
9 subject received the product. 
B. Data Analysis
1. Average Bioequivalence
a. Overview
Parametric (normal-theory) methods are recommended for the analysis of log-
transformed BE measures.  For average BE using the criterion stated in
equations 2 or 3 (section III.A), the general approach is to construct a 90%
confidence interval for the quantity m -m  and to reach a conclusion of average
T R
BE if this confidence interval is contained in the interval [-q  , q ].  Due to the
A A
nature of normal-theory confidence intervals, this is equivalent to carrying out
two one-sided tests of hypothesis at the 5% level of significance (Schuirmann
1987).
The 90% confidence interval for the difference in the means of the log-
transformed data should be calculated using methods appropriate to the
experimental design.  The antilogs of the confidence limits obtained constitute
the 90% confidence interval for the ratio of the geometric means between the T
and R products.
b. Nonreplicated Crossover Designs
For nonreplicated crossover designs, this guidance recommends parametric
(normal-theory) procedures to analyze log-transformed BA measures.  General
linear model procedures available in PROC GLM in SAS or equivalent
software are preferred, although linear mixed-effects model procedures can also
be indicated for analysis of nonreplicated crossover studies. 
For example, for a conventional two-treatment, two-period, two-sequence (2 x
2) randomized crossover design, the statistical model typically includes factors
accounting for the following sources of variation:  sequence, subjects nested in
sequences, period, and treatment. The Estimate statement in SAS PROC
GLM, or equivalent statement in other software, should be used to obtain
estimates for the adjusted differences between treatment means and the
standard error associated with these differences.
c. Replicated Crossover Designs
10 Linear mixed-effects model procedures, available in PROC MIXED in SAS or
equivalent software, should be used for the analysis of replicated crossover
studies for average BE.  Appendix E includes an example of SAS program
statements.
d. Parallel Designs
For parallel designs, the confidence interval for the difference of means in the
log scale can be computed using the total between-subject variance.  As in the
analysis for replicated designs (section VI. B.1.b), equal variances should not
be assumed.
2. Population Bioequivalence
a. Overview
Analysis of BE data using the population approach (section IV.B) should focus
first on estimation of the mean difference between the T and R for the log-
transformed BA measure and estimation of the total variance for each of the
two formulations.  This can be done using relatively simple unbiased estimators
such as the method of moments (MM) (Chinchilli 1996, and Chinchilli and
Esinhart 1996).  After the estimation of the mean difference and the variances
has been completed, a 95% upper confidence bound for the population BE
criterion can be obtained, or equivalently a 95% upper confidence bound for a
linearized form of the population BE criterion can be obtained.  Population BE
should be considered to be established for a particular log-transformed BA
measure if the 95% upper confidence bound for the criterion is less than or
equal to the BE limit, q , or equivalently if the 95% upper confidence bound for
P
the linearized criterion is less than or equal to 0. 
To obtain the 95% upper confidence bound of the criterion, intervals based on
validated approaches can be used.  Validation approaches should be reviewed
with appropriate staff in CDER.  Appendix F includes an example of upper
confidence bound determination using a population BE approach.
b. Nonreplicated Crossover Designs
For nonreplicated crossover studies, any available method (e.g., SAS PROC
GLM or equivalent software) can be used to obtain an unbiased estimate of the
mean difference in log-transformed BA measures between the T and R
products.  The total variance for each formulation should be estimated by the
11 usual sample variance, computed separately in each sequence and then pooled
across sequences.
c. Replicated Crossover Designs
For replicated crossover studies, the approach should be the same as for
nonreplicated crossover designs, but care should be taken to obtain proper
estimates of the total variances.  One approach is to estimate the within- and
between-subject components separately, as for individual BE (see section
VI.B.3), and then sum them to obtain the total variance.  The method for the
upper confidence bound should be consistent with the method used for
estimating the variances. 
d. Parallel Designs
The estimate of the means and variances from parallel designs should be the
same as for nonreplicated crossover designs.  The method for the upper
confidence bound should be modified to reflect independent rather than paired
samples and to allow for unequal variances.
3. Individual Bioequivalence
Analysis of BE data using an individual BE approach (section IV.C) should focus on
estimation of the mean difference between T and R for the log-transformed BA
measure, the subject-by-formulation interaction variance, and the within-subject
variance for each of the two formulations.  For this purpose, we recommend the MM
approach.
To obtain the 95% upper confidence bound of a linearized form of the individual BE
criterion, intervals based on validated approaches can be used.  An example is
described in Appendix G.  After the estimation of the mean difference and the variances
has been completed, a 95% upper confidence bound for the individual BE criterion can
be obtained, or equivalently a 95% upper confidence bound for a linearized form of the
individual BE criterion can be obtained.  Individual BE should be considered to be
established for a particular log-transformed BA measure if the 95% upper confidence
bound for the criterion is less than or equal to the BE limit, q , or equivalently if the 95%
I
upper confidence bound for the linearized criterion is less than or equal to 0. 
The restricted maximum likelihood (REML) method may be useful to estimate mean
differences and variances when subjects with some missing data are included in the
statistical analysis.  A key distinction between the REML and MM methods relates to
12 differences in estimating variance terms and is further discussed in Appendix H. 
Sponsors considering alternative methods to REML or MM are encouraged to discuss
their approaches with appropriate CDER review staff prior to submitting their
applications.
VII. MISCELLANEOUS ISSUES
A. Studies in Multiple Groups
If a crossover study is carried out in two or more groups of subjects (e.g., if for logistical
reasons only a limited number of subjects can be studied at one time), the statistical model
should be modified to reflect the multigroup nature of the study.  In particular, the model should
reflect the fact that the periods for the first group are different from the periods for the second
group.  This applies to all of the approaches (average, population, and individual BE) described
in this guidance.
If the study is carried out in two or more groups and those groups are studied at different clinical
sites, or at the same site but greatly separated in time (months apart, for example), questions
may arise as to whether the results from the several groups should be combined in a single
analysis.  Such cases should be discussed with the appropriate CDER review division.
A sequential design, in which the decision to study a second group of subjects is based on the
results from the first group, calls for different statistical methods and is outside the scope of this
guidance.  Those wishing to use a sequential design should consult the appropriate CDER
review division.
B. Carryover Effects
Use of crossover designs for BE studies allows each subject to serve as his or her own control
to improve the precision of the comparison.  One of the assumptions underlying this principle is
that carryover effects (also called residual effects) are either absent (the response to a
formulation administered in a particular period of the design is unaffected by formulations
administered in earlier periods) or equal for each formulation and preceding formulation.  If
carryover effects are present in a crossover study and are not equal, the usual crossover
estimate of m -m  could be biased.  One limitation of a conventional two-formulation, two-
T R
period, two-sequence crossover design is that the only statistical test available for the presence
of unequal carryover effects is the sequence test in the analysis of variance (ANOVA) for the
crossover design.  This is a between-subject test, which would be expected to have poor
discriminating power in a typical BE study.  Furthermore, if the possibility of unequal carryover
effects cannot be ruled out, no unbiased estimate of m -m  based on within-subject
T R
13 comparisons can be obtained with this design.
For replicated crossover studies, a within-subject test for unequal carryover effects can be
obtained under certain assumptions.  Typically only first-order carryover effects are considered
of concern (i.e., the carryover effects, if they occur, only affect the response to the formulation
administered in the next period of the design).  Under this assumption, consideration of
carryover effects could be more complicated for replicated crossover studies than for
nonreplicated studies.  The carryover effect could depend not only on the formulation that
preceded the current period, but also on the formulation that is administered in the current
period.  This is called a direct-by-carryover interaction.  The need to consider more than just
simple first-order carryover effects has been emphasized (Fleiss 1989).  With a replicated
crossover design, a within-subject estimate of m -m  unbiased by general first-order carryover
T R
effects can be obtained, but such an estimate could be imprecise, reducing the power of the
study to conclude BE.
In most cases, for both replicated and nonreplicated crossover designs, the possibility of
unequal carryover effects is considered unlikely in a BE study under the following circumstances:
!  It is a single-dose study. 
!  The drug is not an endogenous entity.
!  More than an adequate washout period has been allowed between periods of the study
and in the subsequent periods the predose biological matrix samples do not exhibit a
detectable drug level in any of the subjects.
!  The study meets all scientific criteria (e.g., it is based on an acceptable study protocol
and it contains sufficient validated assay methodology).
The possibility of unequal carryover effects can also be discounted for multiple-dose studies
and/or studies in patients, provided that the drug is not an endogenous entity and the studies
meet all scientific criteria as described above.  Under all other circumstances, the sponsor or
applicant could be asked to consider the possibility of unequal carryover effects, including a
direct-by-carryover interaction.  If there is evidence of carryover effects, sponsors should
describe their proposed approach in the study protocol, including statistical tests for the
presence of such effects and procedures to be followed.  Sponsors who suspect that carryover
effects might be an issue may wish to conduct a BE study with parallel designs.
C. Outlier Considerations
Outlier data in BE studies are defined as subject data for one or more BA measures that are
14 discordant with corresponding data for that subject and/or for the rest of the subjects in a study.
 Because BE studies are usually carried out as crossover studies, the most important type of
subject outlier is the within-subject outlier, where one subject or a few subjects differ notably
from the rest of the subjects with respect to a within-subject T-R comparison.  The existence of
a subject outlier with no protocol violations could indicate one of the following situations:
1. Product Failure
Product failure could occur, for example, when a subject exhibits an unusually high or
low response to one or the other of the products because of a problem with the specific
dosage unit administered.  This could occur, for example, with a sustained and/or
delayed-release dosage form exhibiting dose dumping or a dosage unit with a coating
that inhibits dissolution.
2. Subject-by-Formulation Interaction
A subject-by-formulation interaction could occur when an individual is representative of
subjects present in the general population in low numbers, for whom the relative BA of
the two products is markedly different than for the majority of the population, and for
whom the two products are not bioequivalent, even though they might be bioequivalent
in the majority of the population.
In the case of product failure, the unusual response could be present for either the T or R
product.  However, in the case of a subpopulation, even if the unusual response is observed on
the R product, there could still be concern for lack of interchangeability of the two products. 
For these reasons, deletion of outlier values is generally discouraged, particularly for
nonreplicated designs.  With replicated crossover designs, the retest character of these designs
should indicate whether to delete an outlier value or not.  Sponsors or applicants with these
types of data sets may wish to review how to handle outliers with appropriate review staff.
D. Discontinuity
The mixed-scaling approach has a discontinuity at the changeover point, s  (individual BE
W0
criterion) or s  (population BE criterion), from constant- to reference-scaling.  For example, if
T0
the estimate of the within-subject standard deviation of the reference is just above the
changeover point, the confidence interval will be wider than just below.  In this context, the
confidence interval could pass the predetermined BE limit if the estimate is just below the
boundary and could fail if just above.  This guidance recommends that sponsors applying the
individual BE approach may use either reference-scaling or constant-scaling at either side of the
changeover point.  With this approach, the multiple testing inflates the type I error rate slightly,
to approximately 6.5%, but only over a small interval of s  (about 0.18-0.20).
WR
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18 Patnaik, R. N., L.J. Lesko, M.-L. Chen, R.L. Williams, and the FDA Population and Individual
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52.
20 APPENDIX A
Standards
The equations in section IV call for standards to be established (i.e., s  and q  for assessment of
T0 P
population BE, s  and q  for individual BE).  The recommended approach to establishing these
W0 I
standards is described below.
A.  ss  and ss
T0 W0
As indicated in section IV, a general objective in assessing BE should be to compare the
difference in the BA log-measure of interest after the administration of the T and R formulations,
T-R, with the difference in the same log-metric after two administrations of the R formulation,
R-RN.
1. Population Bioequivalence
For population BE, the comparisons of interest should be expressed in terms of the ratio
of the expected squared difference between T and R (administered to different
individuals) and the expected squared difference between R and RN (administered to
different individuals), as shown below. 
E(T - R)2 = (m  - m )2 + s 2 + s 2 Equation 8
T R TT TR
E(R - RN)2 = 2s 2 Equation 9
TR
E(T - R)2      (m  - m )2 + s 2 + s 2
T R TT TR
------------- = -------------------------------
Equation 10
E(R - RN)2                 2s 2
TR
The population BE criterion in equation 4 (section IV.B.) is derived from equation 10,
such that the criterion equals zero for two identical formulations.  The square root of
equation 10 yields the population difference ratio (PDR):
21         (m  - m )2 + s 2 + s 2
T R TT TR
PDR  = [ -------------------------------]2 Equation 11
                           2s 2
TR
The PDR is the square root of the ratio of the expected squared T-R difference
compared to the expected squared R-R' difference in the population.  It should be
noted that the PDR is monotonically related to the population BE criterion (PBC)
described in equation 4 as follows:
 
 
PDR = (PBC/2 + 1)2            Equation 12
Sponsors or applicants wishing to use the population BE approach should contact the
Agency for further information on s .
T0
2. Individual Bioequivalence
For individual BE, the comparisons of interest should be expressed in terms of the ratio
of the expected squared difference between T and R (administered to the same
individual) and the expected squared difference between R and RN (two administrations
of R to the same individual), as shown below.
  E(T - R)2 = (m  - m )2 + s 2  + s 2 + s 2 Equation 13
T R D WT WR
E(R - RN)2  = 2s 2 Equation 14
WR
E(T - R)2            (m  - m )2 + s 2 + s 2 + s 2
T R D WT WR
------------- = ----------------------------------------
Equation 15
E(R - RN)2                      2s 2
WR
The individual BE criterion in equation 6 (section IV.C.) is derived from equation 15,
such that the criterion equals zero for two identical formulations.  The square root of
equation 15 is the individual difference ratio (IDR):
22     (m  - m )2 + s 2 + s 2 + s 2
T R D WT WR
IDR  = [ ----------------------------------------]2 Equation 16
                    2s 2
WR
The IDR is the square root of the ratio of the expected squared T-R difference
compared to the expected squared R-R' difference within an individual.  The IDR is
monotonically related to the individual BE criterion (IBC) described in equation 6 as
follows:
IDR = (IBC/2 + 1)2 Equation 17
This guidance recommends that s  = 0.2, based on the consideration of the maximum
W0
allowable IDR of 1.25.4
B. qq  and qq
P I
The determination of q  and q  should be based on the consideration of average BE criterion
P I
and the addition of variance terms to the population and individual BE criterion, as expressed by
the formula below.
average BE limit + variance factor
q  =  -----------------------------------------------
variance
1. Population Bioequivalence
(ln1.25)2   + e
P
q  = -------------------------- Equation 18
P
         s 2
T0
The value of e  for population BE is guided by the consideration of the variance term
P
(s 2 - s 2) added to the average BE criterion.  Sponsors or applicants wishing to use
TT TR
the population BE approach should contact the Agency for further information on e
P
and q .
P
                    
4
 The IDR upper bound of 1.25 is drawn from the currently used upper BE limit of 1.25 for the average BE
criterion.
23 2. Individual Bioequivalence
(ln1.25)2   +  e
I
 q  =  ----------------------- Equation 19
I
         s 2
W0
The value of e  for individual BE is guided by the consideration of the estimate of
I
subject-by-formulation interaction (s ) as well as the difference in within-subject
D
variability (s 2 - s 2) added to the average BE criterion.  The recommended
WT WR
allowance for the variance term (s 2 - s 2) is 0.02.  In addition, this guidance
WT WR
recommends a s 2 allowance of 0.03.  The magnitude of s  is associated with the
D D
percentage of individuals whose average T to R ratios lie outside 0.8-1.25.  It is
estimated that if s  = 0.1356, ~10% of the individuals would have their average ratios
D
outside 0.8-1.25, even if m  - m = 0.  When s  = 0.1741, the probability is ~20%.
T R D
Accordingly, on the basis of consideration for both s  and variability (s 2 - s 2) in
D WT WR
the criterion, this guidance recommends that e  = 0.05.
I
24 APPENDIX B
Choice of Specific Replicated Crossover Designs
Appendix B describes why FDA prefers replicated crossover designs with only two sequences, and
why we recommend the specific designs described in section V.A of this guidance.
1. Reasons Unrelated to Carryover Effects
Each unique combination of sequence and period in a replicated crossover design can be called a cell of
the design.  For example, the two-sequence, four-period design recommended in
section V.A.1 has 8 cells.  The four-sequence, four-period design below has 16 cells.
 Period
1 2 3 4
1 T R R T
2 R T T R
Sequence
3 T T R R
4 R R T T
The total number of degrees-of-freedom attributable to comparisons among the cells is just the number
of cells minus one (unless there are cells with no observations).
The fixed effects that are usually included in the statistical analysis are sequence, period, and treatment
(i.e., formulation).  The number of degrees-of-freedom attributable to each fixed effect is generally equal
to the number of levels of the effect, minus one.  Thus, in the case of the two-sequence, four-period
design recommended in section V.A.1, there would be 2-1=1 degree-of-freedom due to sequence, 4-
1=3 degrees-of-freedom due to period, and 2-1=1 degree-of-freedom due to treatment, for a total of
1+3+1=5 degrees-of-freedom due to the three fixed effects.  Because these 5 degrees-of-freedom do
not account for all 7 degrees-of-freedom attributable to the eight cells of the design, the fixed effects
model is not saturated.  There could be some controversy as to whether a fixed effects model that
accounts for more or all of the degrees-of-freedom due to cells (i.e., a more saturated fixed effects
model) should be used.  For example, an effect for sequence-by-treatment interaction might be included
in addition to the three main effects  sequence, period, and treatment.  Alternatively, a sequence-by-
25 period interaction effect might be included, which would fully saturate the fixed effects model.
If the replicated crossover design has only two sequences, use of only the three main effects (sequence,
period, and treatment) in the fixed effects model or use of a more saturated model makes little difference
to the results of the analysis, provided there are no missing observations and the study is carried out in
one group of subjects.  The least squares estimate of m -m  will be the same for the main effects model
T R
and for the saturated model.  Also, the method of moments (MM) estimators of the variance terms in
the model used in some approaches to assessment of population and individual BE (see Appendix H),
which represent within-sequence comparisons, are generally fully efficient regardless of whether the
main effects model or the saturated model is used.
If the replicated crossover design has more than two sequences, these advantages are no longer
present.  Main effects models will generally produce different estimates of m -m  than saturated models
T R
(unless the number of subjects in each sequence is equal), and there is no well-accepted basis for
choosing between these different estimates.  Also, MM estimators of variance terms will be fully efficient
only for saturated models, while for main effects models fully efficient estimators would have to include
some between-sequence components, complicating the analysis.  Thus, use of designs with only two
sequences minimizes or avoids certain ambiguities due to the method of estimating variances or due to
specific choices of fixed effects to be included in the statistical model.
2. Reasons Related to Carryover Effects
One of the reasons to use the four-sequence, four-period design described above is that it is thought to
be optimal if carryover effects are included in the model.  Similarly, the two-sequence, three-period
design
     Period
1 2 3
1 T R R
Sequence
2 R T T
is thought to be optimal among three-period replicated crossover designs.  Both of these designs are
strongly balanced for carryover effects, meaning that each treatment is preceded by each other
treatment and itself an equal number of times.
With these designs, no efficiency is lost by including simple first-order carryover effects in the statistical
model.  However, if the possibility of carryover effects is to be considered in the statistical analysis of
BE studies, the possibility of direct-by-carryover interaction should also be considered.  If direct-by-
26 carryover interaction is present in the statistical model, these favored designs are no longer optimal. 
Indeed, the TRR/RTT design does not permit an unbiased within-subject estimate of m -m  in the
T R
presence of general direct-by-carryover interaction.
The issue of whether a purely main effects model or a more saturated model should be specified, as
described in the previous section, also is affected by possible carryover effects.  If carryover effects,
including direct-by-carryover interaction, are included in the statistical model, these effects will be
partially confounded with sequence-by-treatment interaction in four-sequence or six-sequence
replicated crossover designs, but not in two-sequence designs.
In the case of the four-period and three-period designs recommended in section V.A.1, the estimate of
m -m , adjusted for first-order carryover effects including direct-by-carryover interaction, is as efficient
T R
or more efficient than for any other two-treatment replicated crossover designs.
3. Two-Period Replicated Crossover Designs
For the majority of drug products, two-period replicated crossover designs such as the Balaam design
(which uses the sequences TR, RT, TT, and RR) should be avoided for individual BE because subjects
in the TT or RR sequence do not provide any information on subject-by-formulation interaction. 
However, the Balaam design may be useful for particular drug products (e.g., a long half-life drug for
which a two-period study would be feasible but a three- or more period study would not).
27 APPENDIX C
Sample Size Determination
Sample sizes for average BE should be obtained using published formulas.  Sample sizes for population
and individual BE should be based on simulated data.  The simulations should be conducted using a
default situation allowing the two formulations to vary as much as 5% in average BA with equal
variances and certain magnitude of subject-by-formulation interaction.  The study should have 80 or
90% power to conclude BE between these two formulations.  Sample size also depends on the
magnitude of variability and the design of the study.  Variance estimates to determine the number of
subjects for a specific drug can be obtained from the biomedical literature and/or pilot studies.
Tables 1-4 below give sample sizes for 80% and 90% power using the specified study design, given a
selection of within-subject standard deviations (natural log scale), between-subject standard deviations
(natural log scale), and subject-by-formulation interaction, as appropriate.
Table 1
Average Bioequivalence
Estimated Numbers of Subjects
 DD=0.05
80% Power 90% Power
s  = s  2P  4P  2P  4P
WT D
0.15 0.01 12 6 16 8
0.10 14 10 18 12
0.15 16 12 22 16
0.23 0.01 24 12 32 16
0.10 26 16 36 20
0.15 30 18 38 24
0.30 0.01 40 20 54 28
0.10 42 24 56 30
0.15 44 26 60 34
0.50 0.01 108 54 144 72
0.10 110 58 148 76
0.15 112 60 150 80
Note: 1.  Results for two-period designs use method of Diletti et al. (Diletti 1991).
2.  Results for four-period designs use relative efficiency data of Liu (Liu 1995).
28 Table 2
Population Bioequivalence
Four-Period Design (RTRT/TRTR)
Estimated Numbers of Subjects
e  =0.02,  DD=0.05
P
s  = s s =s 80% Power 90% Power
WR WT BR BT
0.15 0.15 18 22
0.30 24 32
0.23 0.23 22 28
0.46 24 32
0.30 0.30 22 28
0.60 26 34
0.50 0.50 22 28
1.00 26 34
Note: Results for population BE are approximate from simulation studies
(1,540 simulations for each parameter combination), assuming two-sequence,
four-period trials with a balanced design across sequences.
29 Table 3
Individual Bioequivalence
Estimated Numbers of Subjects
e  =0.05,  DD=0.05
I
80% Power 90% Power
s  = s 3P 4P 3P  4P
WT D
0.15 0.01 14 10 18 12
0.10 18 14 24 16
0.15 28 22 36 26
0.23 0.01 42 22 54 30
0.10 56 30 74 40
0.15 76 42 100 56
0.30 0.01 52 28 70 36
0.10 60 32 82 42
0.15 76 42 100 56
0.50 0.01 52 28 70 36
0.10 60 32 82 42
0.15 76 42 100 56
Note: Results for individual BE are approximate using simulations (5,000 simulations
for each parameter combination).  The designs used in simulations are RTR/TRT (3P)
and RTRT/TRTR (4P) assuming two-sequence trials with a balanced design across
sequences.
While the above sample sizes assume equal within-subject standard deviations, simulation studies for 3-
period and 4-period designs reveal that if D = 0 and s 2 - s 2= 0.05, the sample sizes given will
WTWR
provide either 80% or 90% power for these studies.
To maintain consistency with section V.C, which suggests a minimum of 12 subjects in all BE studies,
the one case where n = 10 provides 80% power should be increased to n = 12.
.
30 Table 4
Individual Bioequivalence
Estimated Numbers of Subjects
e  =0.05,  DD=0.10
I
With Constraint on DD (0.8  exp(DD)  1.25)
80% Power 90% Power
s  = s 4P  4P
WT D
0.30 0.01 30 40
0.10 36 48
0.15 42 56
0.50 0.01 34 46
0.10 36 48
0.15 42 56
Note: Results for individual BE are approximate using simulations (5,000 simulations
for each parameter combination).  The designs used in simulations are RTRT/TRTR (4P),
assuming two-sequence trials with a balanced design across sequences.  When D =0.05,
sample sizes remain the same as given in Table 3.  This is because the studies are already
powered for variance estimation and inference, and therefore, a constraint on the point
estimate of D  has little influence on the sample size for small values of D .
31 APPENDIX D
Rationale for Logarithmic Transformation of Pharmacokinetic Data
A. Clinical Rationale
The FDA Generic Drugs Advisory Committee recommended in 1991 that the primary comparison of
interest in a BE study is the ratio, rather than the difference, between average parameter data from the T
and R formulations.  Using logarithmic transformation, the general linear statistical model employed in
the analysis of BE data allows inferences about the difference between the two means on the log scale,
which can then be retransformed into inferences about the ratio of the two averages (means or medians)
on the original scale.  Logarithmic transformation thus achieves a general comparison based on the ratio
rather than the differences.
B. Pharmacokinetic Rationale
Westlake observed that a multiplicative model is postulated for pharmacokinetic measures in BA/BE
studies (i.e., AUC and Cmax, but not Tmax) (Westlake 1973 and 1988).  Assuming that elimination of
the drug is first-order and only occurs from the central compartment, the following equation holds after
an extravascular route of administration:
AUC  = FD/CL Equation 20
0-4
                    
             = FD/(VK ) Equation 21
e
where F is the fraction absorbed, D is the administered dose, and FD is the amount of drug absorbed. 
CL is the clearance of a given subject that is the product of the apparent volume of distribution (V) and
the elimination rate constant (K ).5  The use of AUC as a measure of the amount of drug absorbed
e
                    
     5 Note that a more general equation can be written for any multicompartmental model as
                 AUC  = FD/V  l   Equation 22
0-(cid:31) d n
              
where V  is the volume of distribution relating drug concentration in plasma or blood to the amount of drug in the
d
body during the terminal exponential phase, and l  is the terminal slope of the concentration-time curve. 
n
32 involves a multiplicative term (CL) that might be regarded as a function of the subject.  For this reason,
Westlake contends that the subject effect is not additive if the data are analyzed on the original scale of
measurement. 
Logarithmic transformation of the AUC data will bring the CL (VK ) term into the following equation in
e
an additive fashion:
  lnAUC  = ln F + ln D - ln V - ln K              Equation 23
0-4 e
Similar arguments were given for Cmax.  The following equation applies for a drug exhibiting one
compartmental characteristics:
C  = (FD/V) x e-ke*Tmax  Equation 24
max
where again F, D and V are introduced into the model in a multiplicative manner.  However, after
logarithmic transformation, the equation becomes
                  lnC  = ln F + ln D - ln V - KT             Equation 25
max e max 
Thus, log transformation of the Cmax data also results in the additive treatment of the V term.
33 APPENDIX E
SAS Program Statements for Average BE Analysis of
Replicated Crossover Studies
The following illustrates an example of program statements to run the average BE analysis using
PROC MIXED in SAS version 6.12, with SEQ, SUBJ, PER, and TRT identifying sequence,
subject, period, and treatment variables, respectively, and Y denoting the response measure (e.g.,
log(AUC), log(Cmax)) being analyzed:
PROC MIXED;
CLASSES SEQ SUBJ PER TRT;
MODEL  Y = SEQ PER TRT/ DDFM=SATTERTH;
RANDOM  TRT/TYPE=FA0(2) SUB=SUBJ G;
REPEATED/GRP=TRT SUB=SUBJ;
ESTIMATE 'T vs. R' TRT 1 -1/CL ALPHA=0.1;
The Estimate statement assumes that the code for the T formulation precedes the code for the R
formulation in sort order (this would be the case, for example, if T were coded as 1 and R were
coded as 2).  If the R code precedes the T code in sort order, the coefficients in the Estimate
statement would be changed to -1 1.
In the Random statement, TYPE=FA0(2) could possibly be replaced by TYPE=CSH.  This
guidance recommends that TYPE=UN not be used, as it could result in an invalid (i.e., not non-
negative definite) estimated covariance matrix.
Additions and modifications to these statements can be made if the study is carried out in more than
one group of subjects.
34 APPENDIX F
Method for Statistical Test of Population Bioequivalence Criterion
Four-Period Crossover Designs
Appendix F describes a method for using the population BE criterion (see section IV.B, equations 4
and 5).  The procedure involves the computation of a test statistic that is either positive (does not
conclude population BE) or negative (concludes population BE).
Consider the following statistical model which assumes a four-period design with equal replication of
T and R in each of s sequences with an assumption of no (or equal) carryover effects (equal
carryovers go into the period effects)
Y = mg+ d+ e+
ijkl kiklijkijkl
where i=1,Ks indicates sequence,  j =1,Kn  indicates subject within sequence i, k = R, T
i
indicates treatment, l =1, 2 indicates replicate on treatment k for subjects within sequence i.  Y  is
ijkl
the response of replicate l on treatment k for subject  j in sequence i, g  represents the fixed
ikl
effect of replicate l on treatment k in sequence i, d  is the random subject effect for subject  j in
ijk
sequence i on treatment k, and e  is the random error for subject  j within sequence i on
ijkl
replicate l of treatment k.  The e s are assumed to be mutually independent and identically
ijkl
distributed as
e  ~ N(0, s 2 )
ijkl Wk
for i=1,Ks,  j =1,Kn , k = R, T, and l = 1, 2.  Also, the random subject effects
i
( )
dd =+m d +,m d are assumed to be mutually independent and distributed as
ijRijRTijT
 (cid:230) msr(cid:246)s,(cid:230) s 2 (cid:246)
dd ~ N  (cid:231) RBRB(cid:247)TB(cid:231)R (cid:247) .
ij 2o  m   rss s 2 
T BTBRBT
The following constraint is applied to the nuisance parameters to avoid overparameterization of the
model for k=R, T:
35 (cid:229) s(cid:229) 2 g =0
ikl
i=1 l=1
This statistical model proposed by Chinchilli and Esinhart assumes s*p location parameters (where p
is the number of periods) that can be partitioned into t treatment parameters and sp-t nuisance
parameters (Chinchilli and Esinhart 1996).  This produces a saturated model.  The various nuisance
parameters are estimated in this model, but the focus is on the parameters needed for population BE.
 In some designs, the sequence and period effects can be estimated through a reparametrization of
the nuisance effects.
This model definition can be extended to other crossover designs.
Linearized Criteria (from section IV. B, equations 4 and 5):
!  Reference-Scaled:
h =-+(m)(m)s --(cid:215) 2 s<q 0s2 2 2
1 TRTTTRPTR

 
Constant-Scaled:
hm=m-s()() +sq2 s--(cid:215) 02< 2 2    
2 TRTTTRP T 0
Estimating the Linearized Criteria:
The estimation of the linearized criteria depends on study designs.  The remaining estimation and
confidence interval procedures assume a four-period design with equal replication of T and R in each
of s sequences.  The reparametrizations are defined as:
UY = 1Y*( + )
TijijTijT 2 1 2
UY = 1Y *( + )
RijijRijR 2 1 2
VY = 1Y *( - )
TijijTijT 2 1 2
VY = 1Y *( - )
RijijRijR 2 1 2
IY= Y- ,
ijijTijR  
36  for i=1,K,s and  j =1,K,n , where
i
1( ) 1( )
YY = Y + and YY = Y + .
ijTigjTijT2 1 2 ijRigjRijR2 1 2
Compute the formulation means pooling across sequences:
m(cid:217) = 1 (cid:229) s Yk , R =T , and D= m- m
k s i(cid:215)k(cid:215) T R
i=1
where
  Y = 1 (cid:229) ni(cid:229) 1 2 Y .
ik(cid:215)i(cid:215)jkl n 2
i j=1 l=1
Compute the variances of U ,,U ,V V , pooling across sequences, and denote these variance
TijRijTijRij
estimates by MUM,,UMVM, V , respectively.  Specifically,
TR T R
MUU= 1U(cid:229)(cid:229) s ni ( - )2
TTijTin
UT i=1 j=1
MVV= 1V(cid:229)(cid:229) s ni ( - )2
TTijTin
VT i=1 j=1
MUU= 1U(cid:229)(cid:229) s ni ( - )2
RRijRin
UR i=1 j=1
MVV= 1V(cid:229)(cid:229) s ni ( - )2
RRijRin
VR i=1 j=1
nn= n= nn=n =s= (cid:230)(cid:231) (cid:229) s- (cid:246)(cid:247)
IU TU RVV T Ri  
i=1
Then, the linearized criteria are estimated by:
!  Reference-Scaled:
(cid:217) (cid:217) 2
h =D++(cid:215)-+(cid:215)+MUM(cid:215) V0M.5U(1M)V[0.5 q]
1 TT PR R
37 
 
Constant-Scaled:
(cid:217) (cid:217) 2
hq=D+s+(cid:215)-(cid:215)+(cid:215)- M(cid:215)UMVM0.5U(1M)[V0.5 ]
2 TTRRP T 0
95% Upper Confidence Bounds for Criteria:
The table below illustrates the construction of a (1- a)level upper confidence bound based on the
(cid:217)
two-sequence, four-period design, for the reference-scaled criterion, h .  Use a =0.05 for a 95%
1
upper confidence bound.
H = Confidence Bound E = Point Estimate U =(H - E )2
q q q q q
(cid:230) 1(cid:246) 2 (cid:217) 2 U
Htn=M(cid:231) D(cid:217) + (cid:230)(cid:231) 1 (cid:229) s - 1 (cid:246)(cid:247) 2(cid:247) ED = D D
Di (cid:231) I 1- a,n- s s2  (cid:247)
 i=1 
(n- s(cid:215)) E1 MU = E1 U1
H1= T
c
2
n- s,a
()n- s2(cid:215) E 0.5(cid:215) MV = E2 U2
H2= T
c
2
n- s,a
Hr3s = ()n- s3(cid:215)Ers -+(1 qp)M=UERrs 3 Ur3s
c
2
n- s,-1a
()n- s4(cid:215)Ers
Hr4s = Ur4s
c2 -+(1)0q(cid:215)(cid:215).5 = MV4Ers
n- s,-1a p R
H =(cid:229) E +(cid:229) ( U )12
h q q
1
H =(cid:229) E +(cid:229) ( U )12 is the upper 95% confidence bound for h(cid:217) .  Note n= (cid:229) s n , where s is
h q q 1 i
1
i=1
the number of sequences, n  is the number of subjects per sequence, andc2  is from the
i a,n- s
cumulative distribution function of the chi-square distribution with n- s degrees of freedom, i.e.
38 (cid:217)
Pr(c2ns-n s c2a,- )= a.  The confidence bound for h2 is computed similarly, adjusting the
constants associated with the variance components where appropriate (in particular, the constant
associated with MU  and MV ).
R R
H = Confidence Bound E = Point Estimate U =(H - E )2
q q q q q
(cid:230) 1(cid:246) 2 (cid:217) 2 U
Htn=M(cid:231) D(cid:217) + (cid:230)(cid:231) 1 (cid:229) s - 1 (cid:246)(cid:247) 2(cid:247) ED = D D
Di (cid:231) I 1- a,n- s s2  (cid:247)
 i=1 
(n- s(cid:215)) E1 MU = E1 U1
H1= T
c
2
n- s,a
()n- s2(cid:215) E 0.5(cid:215) MV = E2 U2
H2= T
c
2
n- s,a
(ns- E)cs3
Hc3s = c2n- s,-1a -(cid:215) 1 MU=ERcs 3 Uc3s
(ns- E)cs4 -(cid:215) 0.5 =MVEcs 4 Uc4s
Hc4s = R
c
2
n- s,-1a
HE =-(cid:215)(cid:229) + q s U2 (cid:229) ( )12
h qPT 0 q
2
Using the mixed-scaling approach, to test for population BE, compute the 95% upper confidence
bound of either the reference-scaled or constant-scaled linearized criterion.  The selection of either
reference-scaled or constant-scaled approach depends on the study estimate of total standard
deviation of the reference product (estimated by [0M.5UM+V ] (cid:215) 12 in the four-period design).  If
R R
the study estimate of standard deviation is  s , the constant-scaled criterion and its associated
T0
confidence interval should be computed.  Otherwise, the reference-scaled criterion and its
confidence interval should be computed.  The procedure for computing each of the confidence
bounds is described above.  If the upper confidence bound for the appropriate criterion is negative
or zero, conclude population BE.  If the upper bound is positive, do not conclude population BE.
39 APPENDIX G
Method for Statistical Test of Individual Bioequivalence Criterion
Appendix G describes a method for using the individual BE criterion (see section IV.C, equations 6
and 7).  The procedure (Hyslop, Hsuan, and Holder 2000) involves the computation of a test
statistic that is either positive (does not conclude individual BE) or negative (concludes individual
BE).
Consider the following statistical model that assumes a four-period design with equal replication of T
and R in each of s sequences with an assumption of no (or equal) carryover effects (equal
carryovers go into the period effects)
Y = mg + d + e +
ijklkiklijkijkl
where i=1,Ks indicates sequence,  j =1,Kn  indicates subject within sequence i, k = R, T
i
indicates treatment, l=1, 2 indicates replicate on treatment k for subjects within sequence i. Y  is the
ijkl
response of replicate l on treatment k for subject  j in sequence i, g  represents the fixed effect
ikl
of replicate l on treatment k in sequence i, d  is the random subject effect for subject  j in
ijk
sequence i on treatment k, and e  is the random error for subject  j within sequence i on
ijkl
replicate l of treatment k.  The e s are assumed to be mutually independent and identically
ijkl
distributed as
e  ~ N(0, s 2 )
ijkl Wk
for i=1,Ks,  j =1,Kn , k = R, T, and l = 1, 2.  Also, the random subject effects
i
( )
dd =+m d +,m d are assumed to be mutually independent and distributed as
ijRijRTijT
 (cid:230) msr(cid:246)s,(cid:230) s 2 (cid:246)
dd ~ N  (cid:231) RBRB(cid:247)TB(cid:231)R (cid:247) .
ij 2o  m   rss s 2 
T BTBRBT
The following constraint is applied to the nuisance parameters to avoid overparameterization of the
model for k = R, T:
40 (cid:229) s(cid:229) 2 g =0
ikl
i=1 l=1
This statistical model proposed by Chinchilli and Esinhart assumes s*p location parameters (where p
is the number of periods) that can be partitioned into t treatment parameters and sp-t nuisance
parameters (Chinchilli and Esinhart, 1996).  This produces a saturated model. The various nuisance
parameters are estimated in this model, but the focus is on the parameters needed for individual BE. 
In some designs, the sequence and period effects can be estimated through a reparametrization of the
nuisance effects.
This model definition can be extended to other crossover designs.
Linearized Criteria (from section IV. C, equations 6 and 7) :

 
Reference-Scaled:
h =-+(m+mss --(cid:215))2(2 s<q s )2 2 20
1 TRDWTWRIWR

 
Constant-Scaled:
hm=m-+s(+)s( sq s--(cid:215) 22 <) 2 0 2 2  
2 TRDWTWRI W 0
Estimating the Linearized Criteria:
The estimation of the linearized criteria depends on study designs.  The remaining estimation and
confidence interval procedures assume a four-period design with equal replication of T and R in each
of s sequences.  The reparametrizations are defined as:
  IY= Y-
ijijTijR  
TY= Y-
ijijTijT 1 2
RY= Y-
ijijRijR 1 2
for i=1,K,s and  j =1,K,n , where
i
41 1( ) 1( )
YY = Y +   and YY = Y +
ijTigjTijT2 1 2 ijRigjRijR2 1 2
Compute the formulation means, and the variances of I , T , and R , pooling across sequences,
ij ij ij
and denote these variance estimates by M , M , and M , respectively, where
I T R
m(cid:217) = 1 (cid:229) s Yk , R =T , and  D= m- m
k s i(cid:215)k(cid:215) T R
i=1
Y = 1 (cid:229) ni(cid:229) 1 2 Y
ik(cid:215)i(cid:215)jkl n 2
i j=1 l=1
MI ==s(cid:217)I 2 1 (cid:229)(cid:229) s ni- ( )2
Iij iI n
I i=1 j=1
n =nnn== s (cid:230)(cid:231) (cid:229) s - (cid:246)(cid:247)
I TR  i 
i=1
MT==sT 2 1 (cid:229)(cid:229) s -ni ( )2
TWTij i 2n
T i=1 j=1
MR==sR 2 1 (cid:229)(cid:229) s n-i ( )2 .
RWRij i 2n
R i=1 j=1
Then, the linearized criteria are estimated by:  

 
Reference-Scaled:  
(cid:217) (cid:217) 2
  h = D+ M+M 0(cid:215).5(1.-5 + )M(cid:215) q
1 I TI R

 
Constant-Scaled:
(cid:217) (cid:217) 2
  hq=sD+ M+ 0(cid:215).51M.5- (cid:215)- (cid:215) M 2
2 I TR I W 0
and the subject-by-formulation interaction variance component can be estimated by:
42 1( )
s 2 =s 2 - s 2+ s 2
D I 2 WT WR
95% Upper Confidence Bounds for Criteria:
The table below illustrates the construction of a (1- a)level upper confidence bound based on the
(cid:217)
two-sequence, four-period design, for the reference-scaled criterion, h .  Use a =0.05 for a 95%
1
upper confidence bound. 
H = Confidence Bound  E = Point Estimate        U =(H - E )2
q q q q q
(cid:230) 1(cid:246) 2 (cid:217) 2 U
H =(cid:231) D(cid:217) +t (cid:230)(cid:231) 1 (cid:229) s n- 1M(cid:246)(cid:247) 2(cid:247) ED = D D
D (cid:231) 1- a,n- s s2 i I (cid:247)
 i=1 
(n- s(cid:215)) M E = M U
H = I I I I
I c2a,n- s
0.5(cid:215) (n- s(cid:215) ) M E =0.5(cid:215) M U
H = T T T T
T
c2
a,n- s
- (1.5+q )(cid:215) (n- s(cid:215) ) M E = - (1.+5 q (cid:215)) M U
H = I R R I R R
R
c2
1- a,n- s
H =(cid:229) E +(cid:229) ( U )12
h q q
1
where n= (cid:229) s n , s is the number of sequences, andc2  is from the cumulative distribution
i a,n- s
i=1
function of the chi-square distribution with n- s degrees of freedom, i.e. Pr(c2n- s  c2a,n- s)= a.
 Then, H =(cid:229) E +(cid:229) ( U )12 is the upper 95% confidence bound for h(cid:217) .  The confidence bound
h q q 1
1
(cid:217)
for h  is computed similarly, adjusting the constants associated with the variance components where
2
appropriate (in particular, the constant associated with M ).
R
43 H = Confidence Bound  E = Point Estimate        U =(H - E )2
q q q q q
(cid:230) 1(cid:246) 2 (cid:217) 2 U
H =(cid:231) D(cid:217) +t (cid:230)(cid:231) 1 (cid:229) s n- 1M(cid:246)(cid:247) 2(cid:247) ED = D D
D (cid:231) 1- a,n- s s2 i I (cid:247)
 i=1 
(n- s(cid:215)) M E = M U
H = I I I I
I c2a,n- s
0.5(cid:215) (n- s(cid:215) ) M E =0.5(cid:215) M U
H = T T T T
T
c2
a,n- s
- (1.5)(cid:215) (n- s(cid:215) ) M E =- (1.5(cid:215)) M U
H = R R R R
R
c2
1-a,n- s
H =(cid:229) E - q (cid:215) s 2+(cid:229) ( U )12
h q I W0 q
2
Using the mixed-scaling approach, to test for individual BE, compute the 95% upper confidence
bound of either the reference-scaled or constant-scaled linearized criterion.  The selection of either
reference-scaled or constant-scaled criterion depends on the study estimate of within-subject
standard deviation of the reference product.  If the study estimate of standard deviation is  s , the
W0
constant-scaled criterion and its associated confidence interval should be computed.  Otherwise, the
reference-scaled criterion and its confidence interval should be computed.  The procedure for
computing each of the confidence bounds is described above.  If the upper confidence bound for the
appropriate criterion is negative or zero, conclude individual BE.  If the upper bound is positive, do
not conclude individual BE.
This guidance recommends that sponsors use either reference-scaling or constant-scaling at the
changeover point (see section VII.D, Discontinuity).  To test for individual BE, compute the 95%
upper confidence bounds of both reference-scaled and constant-scaled linearized criteria.  The
procedure for computing these confidence bounds is described above.  If the upper bound of either
criterion is negative or zero (eitherH or H ), conclude individual BE.  If the upper bounds of
h h
1 2
both criteria are positive, do not conclude individual BE.
44 APPENDIX H
Variance Estimation
Relatively simple unbiased estimators, the method of moments (MM) or the restricted maximum
likelihood (REML) method, can be used to estimate the mean and variance parameters in the
individual BE approach.  A key distinction between the REML and MM methods relates to
differences in estimating variance terms.  The REML method estimates each of the three variances,
s 2, s 2, s 2, separately and then combines them in the individual BE criterion.  The REML
D WR WT
estimate of s 2 is found from estimates of s 2, s 2, and the correlation, r .  The MM approach
D BR BT
is to estimate the sum of the variance terms in the numerator of the criterion, s 2 + s 2 - s 2,
D WT WR
and does not necessarily estimate each component separately.  One consequence of this difference
is that the MM estimator of s 2 is unbiased but could be negative.  The REML approach can also
D
lead to negative estimates, but if the covariance matrix of the random effects is forced to be a
proper covariance matrix, the estimate of s 2 can be made to be non-negative.  This forced non-
D
negativity has the effect of making the estimate positively biased and introduces a small amount of
conservatism to the confidence bound.  The REML method can be used in special cases (e.g., when
substantial missing data are present).  In addition, the MM approaches have not yet been adapted
to models that allow assessment of carryover effects.
45