| 
| 
j 


rate only under one condition, I have termed this ratio survival index 
in this report and represented it by Sy in algebraic notations. Desvite 
its variability, the index is not a useless statistic. It can be used 
with caution, and a few of its characteristics are worthy of notice 
here: 
(a) If the survival rate in a dynamic calculation fluctuates 
from one year to another, the survival-index value obtained in a time- 
specific computation will fluctuate much more violently. This is 
evident in time-specific computations used in table 1 where all the 
birds were reported shot. Another illustration based on birds re- 
ported found dead is given in table 10. 
Table 10.~—Dynamic Versus Time-specific Computations in a Composite 
Life Table for Herring Gulls 
——— Dynamic Life Table | Time-specific Lite Table 
Actual Sample from Marshall (19)7) (explored here) 
Age 1 Number “Alive at No. of Wort, "Alive at Calculated Mort, 
Interval” Dying Start" Deaths Rate? Start" No. Deaths Rate@ 
O-1 2298 3806 2298 60.4 2298 1729 7502 
l=2 569 1508 569 37.7 569 295 51.8 
2-3 27h 939 27h 29.2 27h 83 30.3 
1/ in years 
2/ per cent per annum 
(ob) Any given value for survival index in a time-specific 
analysis may be the result of a wide combination of mortality rates 
(q, and do) in the dynamic part of a composite life table. For in- 
stance, a survival index of 20 will result, if, in the dynamic analysis, 
the successive mortality rates are 80 per cent and 80 per cent, 75 and 
60 per cent, 60 and 30 per cent, 50 and 30 per cent, and so on. Two 
of the combinations just cited are illustrated in table 1l. 
Table 11.--Vagaries of Survival Index Calculations 

ve at 0. 0 Mortality urviva urviva 
Year Start Deaths Rate Rate Index 
1 1000 750 754 25% 20% 
2 250 150 60% LOZ 20% 
3 100 30 30% 70% 
Here Sy = do = 150 ie dy = 30 i 
= q i ~ f 
d, 750 207 and qd, TSO 20% 
Table 12 further illustrates some of the combinations 
possible, 
(c) It should be noticed that certain combinations of 
decreasing dynamic mortality rates can result in constant values for 
37 
