Self assessment questions (cont.)
Question 2
Revision information
The beta parameter of the logit life table system determines the slope of the survivorship function. Values of beta greater than 1.0 indicate that mortality at older ages is more severe than in the standard life table, whilst mortality at younger ages is more favourable, so that the survival values slope steeply with age. Values of beta less than 1.0 indicate that mortality at younger ages is more severe than in the standard life table, whilst mortality at older ages is more favourable, so that the survival values have a flatter slope with age.
Figure 2a shows the relationship between beta and the mean age of a stable population, Figure 2b shows the relationship between beta and the growth rate of a stable population. In all the stable populations described in these figures the GRR that determines the fertility level is 2.0, and the level of mortality is such that life expectancy is just over 50 years.
Choose from the alternatives to explain what these figures show:
1. A gentle mortality slope produces a stable population with an age structure since the underlying life table implies that relatively people survive to old age.
A gentle mortality slope indicates that mortality at older ages is more favourable. Therefore, the age structure will be older since relatively many people will survive to old age.
Correct, well done!
A gentle mortality slope indicates that mortality at older ages is more favourable. Therefore, the age structure will be older since relatively many people will survive to old age.
Yes, correct!
2. A steep sloping mortality life table will be associated with growth rates because there will be a proportion of women in the younger ages associated with higher fertility rates.
Figure 2b shows that growth rates are faster if mortality slope is steep (beta >1); faster growth rates ensure younger age structures with higher proportion of women in the reproductive ages.
Yes, correct!
Correct!
Figure 2a shows that age structure is younger if mortality slope is steep (beta >1); a younger age structures means there will be higher proportion of women in the reproductive ages.
3. With moderate values of fertility and mortality levels, varying the slope of mortality across the ranges typically observed in human populations produce shrinking stable populations.
The gradient of the mortality slope has a fairly large impact on the age structure of stable populations as the shape of the life table reinforces the modest impact of mortality slope on the growth rate. The direct impact of mortality slope on the growth rate is not large enough to cause the growth rate to fall below zero when fertility is moderately high and mortality levels are not high.
Correct, excellent!