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NATIONAL BUREAU OF STANDARDS REPORT

31*93

STATISTICAL ANALYSIS OF EXPERIMENTAL PARACHUTE TEST DATA

Do Teichroew

<nbs£>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1101-40~£L21 June 30* 1954 3493

STATISTICAL ANALYSIS OF EXPERIMENTAL PARACHUTE TEST DATA''"

D. Teichroew

National Bureau of Standards

3

Final report on Project 1101-L|.0-5>1 21/54-7

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

_ , „ Approved for public release by the

The publication, rep r

unless permission is Director of the National Institute of

25.D.C. Suchperr Standards and Technology (NIST)

cal ly prepared If th On October 9, 2015

n part, Is prohibited

tndards, Washington

)ort has been speclflport for Its own use.

' Io Introduction

This report is the result of an investigation conducted under

our Contract 1!01“U0H?131/5U"7 (Navy Contract No 0 NPOLA NASSD~719)°

This contract was set up to provide "services to assist the Navy

Parachute Unit by conducting a study of the results to be gained

through statistical analysis of experimental parachute test data."

The material for this report was obtained by the author in a visit to

El Centro, January n to V~> 9 195>h°

II. Recommendations

1. The major application of statistical theory to the work of

the parachute testing facility should be in the design of the experiment s5 particularly those experiments which are conducted to determine

the best one among a number of parachute types „ The use of designed

experiments should immediately provide more information about, and a

better understanding of, factors which cause variability in parachute

testing data. Eventually the accumulation of such information should

permit statistically significant results to be obtained with smaller

experiments

2» The application of designed experiments would not cause any

drastic change in the manner in which experiments are carried out at

present o In fact, the experiments discussed later in this report

would require only minor refinements to provide additional informat ion

3. Statistical analysis is concerned with random errors and

cannot handle non-random errors „ The data handling procedure should be
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changed so that measurements are not so susceptible to human error0

Another source of non-random error lies in the lack of precise definitions of concepts,, for example,, the recommendations from the reserve

parachute system experiment are considerably affected by the

definition of a failure <>

I4.0 Inasmuch as the amount of statistical work at the parachute

testing facility is5 at present^ insufficient to occupy a statistician

full time it is recommended that the facility acquire continuous consulting services to design and analyze experiments 0 Hie Discussion of Experiments

In the following discussion we shall make use of several statistical

concepts o These concepts are explained in all standard texts and we need

not take space here to describe therm For the sake of having something

definite we shall use the book "Introduction to Statistical Analysis^"

by Dixon and Massey? published by McGraw Hill in 195>X° In particular

we shall use the technique known as "Analysis of Variance" described in

Chapter 105 pp 0 119“1 and the concept of a binomial distribution

which is described on pp e 191=196

The material and data used in the following discussion is taken

from the reports describing the experiments

lo Analysis of Data for Project TED ELC AE-521U

lo Table I indicates (i) that there is no essential difference

in opening times between A and B^, (ii) that there seems

to be a consistent difference between B and C and that B

is consistently lower than Co
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2o The one discrepancy is that on the 11th C is lower than

Bo The opening time for this day had to be taken from

movies taken at 200 f 0 p 0 So The timing on this camera is

considered unreliable and therefore the data must be

treated with caul ion o It should be pointed out, however,

that the opening times as taken by stop watches, also

indicate that C was less than B on the 11th 0

3o Table II gives the analysis of variance to test whether

the difference between B and C is significant for the 20

drops at speed D for which reliable data is available

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The observed F value is significant at the 5% levelo

From the analysis of variance it may be seen that both

the day effect and the day by parachute type interaction

are non-s ignif leant » A t-test, with the combined error

estimate with 18 degrees of freedom, would therefore yield

the same conclusion as the F testo

iio Table IU gives the analyses of variance for C and B to

test whether there is any difference between two parachutes

of the same type » The tables clearly indicate that the

variation between the two chutes of each type are very

small

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5>o The conclusion to be drawn from the fact that within-type

variation is small is that no greater precision would be

obtained by taking 5 drops of each of U chutes of each

type than by making 10 drops of each of 2 chutes of each
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type ,, as was made « On the other hand, no smaller

precision would be obtained by using It chutes of each

type and it might inspire more confidence that the

chutes are a representative sample of their type*

60 The opening time of C at speed E indicates an erratic

behaviore An examination of the motion pictures of the

drops showed that each of the two parachutes tested had

a deployment delay 0 These delays did not occur on the

same day and therefore there is not immediate assignable

cause for the delay

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Table I - Opening Time Totals for Two Parachutes of Each Type

Parachute Type

Day A B C Speed

n 3o6l In OU 3o65 D

Xh 1+o78 ho79 5»oh D

16 h°92 ho 83 5o21 D

17 Uo8? ho20 ho91 D

18 Uo6l ho6? 5 o 08 D

18 - h<>70 5ohh D

21 - 5c5o 6o00 E

29 in 75 h<>92 6o37 E

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Table Ho Analysis of Variance to Test for Difference

Between Types of Parachutes

dofo SS MoSo F

Days h .1623 oOlj.05

Type of Parachute i o3h0Q <,3100 5°973

Days x Type h o0U6? 0 OII5

Error 10 o5190 o05l9

Table IHo Analysis of Variance to Test for

Differences Between Parachutes of the Same Type

C B

dof 0 SS dof 0 SS

Days h o0807 Days h 0128U

Parachutes 1 o00l5 Parachutes 1 o0203

Days x Parachutes k ol350 Days x Parachutes k o3222

2 o Analysis of TED ELC 525036 Data

The objectives of this experiment as stated in the introduction

ares to ascertain their [four different personnel parachute canopy

configurations] comparative performance with respect to stability., rate

of descent, opening shocks and reliability

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98 The data available could

be analyzed statistically only for rate of descent and opening time.

From an examination of Table I - 3V of Appendix B9 it is clear that

any such analysis would depend on a few values much different from the
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others o For example,, canopy 1, 26 ft. dia. - 22 Gore Conical Type,

was dropped 5 times during the test and the opening times were 1.75,

lo8£, 3.2, lo8, lo?5o It seems unreasonable to accept the 3o2 value

as measuring the same thing as the other values^ either it was

measured incorrectly or something happened to the parachute on this

drop. Unfortunately movies are not available to decide the question.

It would be more meaningful to report the result of these five drops

like this

i

The average opening time is 2f(l<>75 + 1.85 + 1.8 + lo?5)

and in one out of five drops something happened which

Clearly if the first method of reporting is used, comparison between

types of parachutes will be affected by whether such delays occur or

not „

The live jump program reported in Appendix D is probably the

outstanding example of a place where a designed experiment could have

yielded more conclusive results. In its present form the data presented

in Appendix ”0” cannot be analyzed statistically! the data is too

incomplete, e.go, not even the number of jumps with each type of

parachute is given. The use of ranking preference tests has increased

greatly in recent years and all experience indicates that the tests must

be very carefully designed to avoid bias. Every person has prejudices

than to say merely?

The average opening tine is

+ lo?5)o

^(1.75 + lo85 + 3o2 + lo8
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and they may easily affect his judgment „ Xn most tests the judge is not

told which type of product he is testing] in parachute testing this is

clearly impossible and therefore extra care must be taken to avoid bias.

It has also been shown that people cannot discriminate between

many objects if they are not much different , In particular, people are

better at ranking two objects than in ranking three. The more objects

there are the more people tend to forget what the earlier ones were

like o It is, therefore, important to randomize the order in which objects

are testing. The conflicting testimony listed in Appendix D shows that

the jumpers tend to forget, after a large number of jumps, what their

experience in the first few jumps were,

3o Analysis if TED ELC 5 20 6 Data

The major question in the analysis of this data iss what is

meant by the failure of a system? On page 6 of report 5“53 appears the

statement "It is noted that initial tests conducted in which the 200-inch

line was employed resulted in a relatively low terminal velocity of the

test assembly] thereby preventing eoiqplete deployment of the reserve

parachute when released from its pack. Employment of the 200-in.ch reefing line was discontinued on all subsequent tests, 000 " However the

same situation, i,e,, incomplete deployment ©f the reserve pack, was

counted as a failure when it occurred with other reefing lines. There

does not seem to be any reason why this should be counted against the

system.

In listing "percent reliability8' figures, as in Appendix C, Table 3, the number of tests involved in each figure should be given. Since
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percent reliability is a binomial variate its variance can be computed

once the number of tests is known 0 Tests of significance of the difference between two binomial variates can be made by the method given by

Dixon and Massey, p„ 193

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The failure of the attempt to correlate percent reliability and

opening time is not surprising 0 In the first place, the percent reliability figures are ambiguous, as mentioned above « To, the second place,

the opening time measurements proved very inaccurate when compared

with motion pictures ° But most important is the fact that opening

times were not divided intp groups, namely those associated with tests

which were successful and those which were not successful

This eixperiment could also have profited from a scientific design

A start was madej the report, page 6 contains the statement, "It was

believed that by testing the same parachutes without and then with

pocket bands, a more significant apd accurate comparison could be

obtained between the two reserve parachute configurations » " This

belief is certainly correct, but no increase in significance can be

obtained if the fact that the same parachutes were used is not used in

the analysis, and in the analysis this fact is not usedo The figure that

should have been given iss how many reserve systems worked with pocket

bands which did not work without them

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