I__. - International Military Testing Association

2E
24
20
I@
12
8
4
Figure 4
Mean Life Comparison
monlna .- --..- - _._ ___ _.____ . _ _. _ __ _ -.-.- .-. -----
0
12 18 20 24 28 32 38 40 44 48 52 68 to 84 T’
months
--- Inalude censors --O 0m1 iill cansors
Figures 3 and 4 show a'comparison between the data base with
all 1000 airmen (event history analysis) and the data base with
700 airmen (i.e., all censored data omitted). The difference in
the two survival functions (figure 3) is greatest at the 48th
month, the point at which censoring is heaviest.
The difference between the two mean life residual functions
(Figure 4) is greatest at the beginning of the 13th month, basically
because excluding the 300 censored data points removes some
information about how long a task is performed. At the 48th
month the two curves become very similar. Thus, censoring after
the first term has less of an effect on the mean life residual
function.
This data could also be presented in a table format. A portion
of these functions is shown in Table 1.
Month
8;
38
39
40
Table 1
Comparison Data
SUrVlval Function Mean Life ReddUal
lndude Omit include cmt
Censora Coneore Censor8 ceneora
A44 -377 13.307 9.273
-627 -364 13.264 8.871
A16 .a40 12.647 8.2U
.496 .a13 12.078 7.969
A79 .293 11.472 7.602
Discussion
The results of this study show that event history analysis
can be used to investigate task perishability. Due to the method
of collecting task data in the Air Force's Occupational Survey
Program, accurate figures can be obtained for the change in state
of the binary variable, e.g., task perishability. Historical
attrition data are available for all career fields. Thus, censoring
is the only unknown variable, and it can be accurately
estimated by combining occupational and attrition data. Therefore,
an appropriate data base can be created for any AFSC.
The results of the analysis also show the advantage of using
event history to analyze task perishability. Figures 3 and 4
vividly illustrate the difference in analyzing task perishability
141
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