Epidemiologic Investigation of Health Effects in Air Force Personnel ...

eing preferred. If for example, the mortality rate in C 1 is 0.10, then an
unaccountability rate of 0,01 could alter the mortality rate by as much as
10%. Whatever the unaccountability rates, the pattern of unaccountability
must also be compared between groups R 1 and C'. For example, the possibility
of age differences must be examined, particularly if the unaccountability
rates are high. The following paragraphs discuss the analysis of mortality
under the assumption that low unaccountability rates have rendered the mortality
analysis meaningful.
Multiple mortality assessments will be accomplished during
the course of this study, one at the beginning of the study, using available
mortality data on the basic mortality cohorts in C 1 and R 1 (5:1 ratio), and
others using mortality data on R 1 and all controls used in the study (both C 1
and replacements) as controls accumulate prospectively. The procedures
described here will be used in all of these assessments.
Henceforth, within the protocol, the term "mortality data"
does not distinguish between that data collected initially and that data collected
in the future.
The mortality data will be analyzed using several different
approaches. Crude age-specific death rates will first be calculated and
tabulated. Age will be d-jvided into k strata, and person-years will be
observed for each strata as will be the number of deaths in each strata. In
this manner a tabular display will be developed as shown in Table 7.
Table 7
STRATIFIED FORMAT OF AGE-SPECIFIC DEATH
RATES
Ranch Hand
Controls
Person
Years
Deaths
Death
Rate
Person
Years
Deaths
Death
Rate
1
2
PH
Pl2
m 12
P 2 i
P 22
m 22
P21
r 22
PIS
m !3
m 23
P23
"Ilk
•"ik
Since the death rates TIJ and r 2 j are Poisson variables, they
can be contrasted directly. If the relationship of rjj to r 2 j is found to
be consistent between age strata (within statistical variability), a summary
mortality index may be calculated. One summary index that will be calculated
is the Standardized Mortality Ratio (SMR) which is (Armitage, 1971):
32