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### Technical Notes

The Schools and Staffing Survey (SASS), an integrated survey of American schools, school districts, principals, teachers, and student records, is funded by the National Center for Education Statistics (NCES) of the U.S. Department of Education. First conducted during the 1987-88 school year, SASS�is designed to provide periodic data on public and private schools in the United States. Since the 1990-91 school year, SASS has also included Indian schools supported by the Bureau of Indian Affairs, U.S. Department of the Interior. Major categories of data collected in SASS include the characteristics of schools and principals, school programs and policies, and the opinions and attitudes of principals about policies and working conditions.
The analytical power of the data is enhanced by the ability to link survey data for individual local education agencies (LEAs), schools, principals, teachers, and, since the 1993-94 school year, student-level records. The use of comparable questions in each round of SASS makes it possible to monitor changes in the nation�s educational system. The first SASS was administered during the 1987-88 school year, with a teacher followup survey conducted during 1988-89. The two subsequent SASS administrations were at three-year intervals (1990-91 and 1993-94). The next SASS round (1998-99) and subsequent administrations are planned for five-year intervals.
The 1993-94 SASS consisted of separate surveys administered simultaneously to linked samples of respondents./1 These instruments included:
- Teacher Demand and Shortage Questionnaire for Public School Districts (LEAs) , which collected information on student enrollment and district programs and policies from public school districts;
Public, Private, and Indian School Principal Questionnaires
, which collected information on principals� demographic characteristics, education, experiences, compensation, and perceptions of school problems;

Public, Private, and Indian School Questionnaires
, which collected information on school programs and policies, enrollment by grade, student demographic characteristics, and measures of school type;/2

Public, Private, and Indian School Teacher Questionnaires
, which collected information on teachers� education and training, teaching assignments, experience, certification, perceptions and attitudes about teaching, and workplace conditions;

Student Records Questionnaire
, which collected student records data from a subsample of students from surveyed schools on demographic information, current enrollment status, educational activities, support services received, and student performance measures (e.g., GPA); and 

Teacher Followup Survey
, which surveyed a sample of teachers one year after the SASS administration, oversampling those who have�left the profession, and collected data on activities and plans, attitudes about teaching, and job satisfaction.

The analyses for this report on public and private school principals use five SASS instruments: the Public School, Private School, Public School Principal, and Private School Principal Questionnaires, and the Teacher Demand and Shortage Questionnaire for Public School Districts. Data collected with the Student Records Questionnaire and the Teacher Followup Survey were excluded from analyses. Additionally, since less then 0.1 percent of American students attend Indian schools operated by the Bureau of Indian Affairs, these schools (170 schools in 1993-94), were excluded from this report./3
Overview of the Design of SASS
Schools and Staffing Survey continues to be the largest and most thorough national integrated survey of districts, schools, principals, and teachers ever undertaken in this country. The target populations for the SASS surveys include elementary and secondary schools, principals and classroom teachers in these schools, former teachers, and the LEAs that are responsible for administering the public schools. The 1993-94 sample consisted of approximately 9,000 public schools and 3,000 private schools.
Evolution of the SASS Design
The first administration of SASS in 1987-88 integrated three existing NCES survey programs: the Teacher Demand and Shortage Survey, the Public and Private School Surveys, and the Teacher Surveys. The 1987-88 SASS included a public school sample of 9,317 schools selected from the Quality Education Data (QED) file of public schools. The private school sample included 3,513 schools selected primarily from the QED file of private schools supplemented with private school association lists and targeted area samples from telephone directories.
Since that first administration, NCES has implemented a number of changes in the survey design and context to improve study estimates and to better reflect changes in the educational environment. Some of the most important changes that relate to this report are highlighted below:/4
- Beginning with the 1990-91 SASS, the sampling frame for public schools was the NCES Common Core of Data (CCD), an annual census of LEAs and schools. For private schools, the sampling frame for the 1990-91 SASS was the NCES 1989-90 Private School Universe Survey, augmented with state lists and private school association lists. The frame for the 1993-94 SASS was the augmented 1990-91 Private School Universe Survey.

Notably for public schools, the QED and CCD data sources apply slightly different definitions of the school unit. The QED file defined schools in terms of their physical location; the CCD file used for subsequent SASS surveys described schools as "administrative units with principals." Thus, in instances where multiple schools share a single campus, the estimated number of schools increases using the CCD definition.
- Separate School Administrator Questionnaires were developed for public and private school principals in the 1990-91 SASS. In 1993-94, the instrument was renamed to refer to principals instead of school administrators. 
Since 1987-88, a number of revisions to the School Principal Questionnaires have been implemented. Question formats (e.g., the general education background of principals in the 1987-88 and 1990-91), item selection (e.g., lists of principal perceived school problems have grown over time), and item wording (e.g., years of experience before becoming principal) have changed since 1987-88. We do not attempt to describe these changes here; however, these occurrences are noted in the report text, where appropriate.

The measure of community type or "urbanicity" for the 1987-88 SASS involved administrator self-reports. Since the release of the 1987-88 SASS data, the locale code defined for the 1990-91 and 1993-94 SASS was added to the 1987-88 data; this report�s analyses use this new measure of community type. In 1990-91 and 1993-94, community type was derived from a locale code based on school mailing address mapped to Bureau of the Census population density data, Standard Metropolitan Statistical Area (SMSA) codes, and Census codes for urban and rural areas. Analyses of 1987-88 data were conducted using locale code-based urbanicity measures as defined for the 1990-91 and 1993-94 SASS.

All school administrators in 1987-88 responded to items about free/reduced price lunches, an item frequently used as a proxy for school wealth. However, the 1990-91 Private School Questionnaire omitted the free/reduced-price lunch item. We excluded this measure from analyses for private schools.

All data files have been adjusted for complete refusals. For individual questionnaire items that should have been answered but were not, values for the 1987-88,/5 1990-91,and 1993-94 SASS were imputed using data from (1) other questionnaire items, (2) related SASS questionnaires, (3) the sampling frame, or (4) a similar respondent.

Sample Selection
The initial sampling units for SASS were schools./6 The sampling structure was designed to provide separate data for public and private schools, with detail by state for the public sector and by private school association for the private sector. After schools were selected, each public and private school in the sample was sent a letter requesting that school personnel provide a list of all teachers in the school. The returned lists, supplemented by telephone followup, served as the sampling frame for the teacher survey. The same sample was used for the public and private school principal survey. Each LEA that administered one or more of the sample schools in the public sector became part of the sample for the Teacher Demand and Shortage Questionnaire.
Selection of schools. Since the 1990-91 SASS, the public school sampling frame has been the CCD file. The CCD is based on census data collected annually by NCES from state education agencies and is believed to be the most complete list of public schools available. The frame includes regular public schools, military base schools operated by the Department of Defense, and nonregular schools such as special education, vocational, and alternative schools. The public school sampling frame for the 1987-88 SASS was the school file developed by QED.
For private schools in the 1987-88 SASS, the QED private school frame was supplemented with lists obtained from several private school associations and by an area sample of 123 counties or groups of counties in which telephone directories, government offices, and other local sources were utilized to identify omitted private schools. The sampling frame for private schools in the 1990-91 SASS was the NCES 1989-90 Private School Universe Survey, augmented with state lists and private school association lists./7  The 1993-94 SASS private school frame consisted of the 1991-92 Private School Universe Survey, augmented with private school association lists and lists from an area frame./8
Selection of local education agencies. All LEAs that had at least one school selected for the school�sample were included in the LEA sample for the Teacher Demand and Shortage Questionnaires.
Survey Operations Procedures
Survey operations for the 1987-88, 1990-91, and 1993-94 SASS, including sample selection, data collection, and data processing, were carried out under an interagency agreement by the U.S. Bureau of the Census, according to specifications provided by NCES. At the start of each school year, introductory letters containing a Teacher Listing Sheet were mailed to sample schools. These Teacher Listing Sheets, designed to enumerate the instructional staff at each school, served as the sampling frame for the teacher sample. Shortly after the listing sheets are distributed, Teacher Demand and Shortage Questionnaires were mailed to the local education agencies representing the sampled public schools and School Principal Questionnaires were sent to the principals of�the selected public and private schools. School Teacher Questionnaires for teachers selected from lists provided by the sample public and private schools were also mailed at that time. Completed questionnaires were returned by mail to the Census Bureau. Telephone followup interviews of nonrespondents to the questionnaires were conducted by Census Bureau field representatives.
Weighting
Weights of the sample units were developed to produce national and state estimates for teachers, administrators, schools, and local education agencies./9  The basic weights were the inverse of the probability of selection. The weights were also adjusted for nonresponse and to ensure that sample totals (based on responding, nonresponding, and out-of-scope cases) were comparable to the frame totals.
Standard Errors
The estimates presented in the text and tables of this report are based on samples and are subject to sampling variability. Standard errors were estimated using a balanced repeated replications procedure that incorporated the design features of this complex sample survey./10  The standard errors indicate the accuracy of each estimate. If all possible samples of the same�size were surveyed under the same conditions, an interval of 1.96 standard error units below to 1.96 standard error units above a particular statistic would include the true population�value in approximately 95 percent of the cases. Note, however, that the standard�errors do not take into account the effects of biases due to item nonresponse, measurement error, data processing error, or other possible systematic errors. Standard errors for the estimates presented in the text and tables of this report are included in appendix B.
Accuracy of Estimates
Some principals, schools, and districts did not return questionnaires, which resulted in missing data. These missing data, however, should have relatively little impact on the estimates of percentages, means, and counts that this report presents because of nonresponse adjustment strategies employed by SASS./11  Where analyses required examining data across questionnaires (e.g., when analyses included variables from both the Principal Questionnaire and the Teacher Demand and Shortage Questionnaire for Public School Districts), district nonresponse reduced the sample size of respondents because principals located in districts that did not return a questionnaire could not be included in the analyses. Thus, in these cases, the principal totals, and all other estimates presented, were based on a subset of the total number of principals.
The accuracy of any statistic is determined by the joint effects of sampling and nonsampling errors. Both types of error affect the estimates presented in this report./12
Nonsampling Error
Both universe and sample surveys are subject to nonsampling errors. Two types of nonsampling errors occur�nonobservation error and measurement error�and both are extremely difficult to estimate.
Nonobservation error may be due to noncoverage, which occurs when members of the population of interest are excluded from the sampling frame and, therefore, are not included in the survey sample. Nonobservation error also occurs when sampled units (for example, teachers or administrators) refuse to answer some or all of the survey questions. These types of�error are referred to as questionnaire nonresponse (where the entire questionnaire is missing) and item nonresponse (where only some items of the questionnaire are missing). Sample weight adjustment techniques were used to compensate for questionnaire nonresponse; imputation procedures were used to compensate for item nonresponse in SASS./13
Measurement error occurs when mistakes are made when data are edited, coded, or entered into computers (processing errors), when the responses that subjects provide differ from the "true" responses (response errors), and when measurement instruments fail to measure�the characteristics they are intended to measure. Sources of response errors include differences in the ways that respondents interpret questions, faulty respondent memory, and mistakes respondents make when recording their answers. Because estimating the magnitude of these various types of nonsampling errors would require special experiments or access to independent data, information on the scope of these errors is seldom available.
Sampling Error
Sampling error occurs when members of a population are selected (sampled), and only sample members respond to survey questions. Estimates that are based on sample responses will differ somewhat from the data that would have been obtained if a complete census of the relevant population had been taken using the same survey instruments, instructions, and procedures. The estimated standard error of a statistic is a measure of the variation due to sampling and can be used to examine the precision obtained in a particular sample. All estimates and standard errors were calculated using a balanced repeated replications variance estimation program developed to calculate standard errors based upon complex survey designs.
Response Rates and Imputation
The final weighted questionnaire response rates are reported in table C1 for the various SASS years. Table C2 provides the item-response rates for the SASS instruments by year. Values were imputed for items with missing data by (1)�using data from other items on the questionnaire or a related component of the SASS (e.g., a school record to impute district data); (2)�extracting data from the sample frame such as the CCD; or (3)�extracting data from a respondent with similar characteristics./14
The reader should note that all data on principals in this report are imputed. For earlier reports, imputed data on principals for 1987-88 were not available; unimputed data were used. Thus, differences may exist between the 1987-88 estimates reported here and those in previous reports.
Statistical Procedures
The comparisons in the text were tested for statistical significance to ensure that the differences are larger than might be expected from sampling variation. These statistical tests were based on Student�s t statistic. Generally, whether a difference is considered significant is determined by calculating a t value for the difference between a pair of means or percentages, and comparing this value to published tables of values at certain critical levels, called alpha levels. The alpha level is an a priori statement of the probability of inferring that a difference exists when, in fact, it does not (i.e., the observed difference results from sample variation rather than a �true� difference between two means).
----------------------------------------------------------------------------Table C1.�Weighted and unweighted percent response rates by SASS          instrument: 1987-88, 1990-91, and 1993-94----------------------------------------------------------------------------                                   Unweighted                Weighted                           -------------------------------------------------Questionnaire                1987-88 1990-91 1994-94    1987   1990   1993                                                         -88    -91    -94----------------------------------------------------------------------------Teacher demand and shortage for public school districts   89.4    93.7    93.1     90.8   93.5   93.9Public school principal        94.2    96.9    96.6     94.4   96.7   96.6Private school principal       81.2    91.1    90.3     79.3   90.0   87.6Public school                  91.9    95.0    92.0     91.9   95.3   92.3Private school                 79.6    85.1    84.1     78.6   83.9   83.2Public school teacher*         86.5    91.5    88.9     86.4   90.3   88.2Private school teacher*        77.0    83.1    80.6     79.1   84.3   80.2----------------------------------------------------------------------------* The response rates for public and private school teachers exclude theschools that did not provide teacher lists. The overall or effectiveresponse rates for public school teachers, including those that could notbe sampled from nonresponding schools, were 83 percent, 86 percent, and 85percent, respectively, for the 1987-88 through 1993-94 SASS. Overallresponse rates for private school teachers were 70 percent, 75 percent, and73 percent for the SASS administration.----------------------------------------------------------------------------
In order to make proper inferences and interpretations from the statistics, several points�must be kept in mind. First, comparisons resulting in large t statistics may appear to merit special note. However, this is not always the case because the size of the t statistic depends not only on the observed difference in means or percentages being compared, but�also on the standard error of the difference. Thus, a small difference between two groups with a much smaller standard error could result in a large t statistic, but this small difference is�not necessarily noteworthy. Second, when multiple statistical comparisons are made on the same data, it becomes increasingly likely that an indication of a population difference is erroneous. Even when there is no difference in the population, at an alpha level of .05, there is�still a 5 percent chance of concluding that an observed t value representing one comparison in the sample is large enough to be statistically significant. As the number of comparisons increases, so does the risk of making such an error in inference.
To guard against errors of inference based upon multiple comparisons, the Bonferroni procedure to correct significance tests for multiple contrasts was used. This method corrects the significance (or alpha) level for the total number of contrasts made with a particular classification variable. For each classification variable, there are (K*(K-1)/2) possible contrasts (or nonredundant pairwise comparisons), where K is the number of categories. For example, race-ethnicity has five categories (i.e., American Indian/Alaska Native, Asian/Pacific Islander, Hispanic, and White and Black non-Hispanic). With K=5, there are 5*(5-1)/2 or 10 possible comparisons among the race-ethnicity categories. The Bonferroni procedure divides the alpha level for a single t test by the number of possible pairwise comparisons in order provide a new alpha that is corrected for the fact that multiple contrasts are being made.
------------------------------------------------------------------------------------------------------------------------------Table C2.-Unweighted item-response rates for SASS questionnaires, by year                                                             Percent of items with response  Percent of items with a responseQuestionnaire                Range of item-response rates         rate ³ 90 percent               rate PRE>--------------------------------------------------------------------------------------Table C3.-Decision rules for estimates suppresion--------------------------------------------------------------------------------------***For Total Columns:***1. If n £ n ³ 20%, then suppress all totals and counts;3. If n ³ 30, then report all totals and counts.***For Percentages and Proportions:***1. If nden³ 30, then do   A. If nnum£ 2, then suppress percentage;   B. If nnum > 2, then report percentage;2. If 10 dennum£ 30, then suppress percentage;   B. If nnum > 2, then do      1. If C.V. of denominator ³ 20%, then suppress;3. IF ndenPRE>
The formulas used to compute the t statistic was as follows: