Changes in Human Performance with Age 157 



its present state cannot be fully understood without some 

 reference to past events and we can never assume that it is the 

 same on two different occasions — indeed we can be sure it is 

 not. For many purposes these variations may be unimportant, 

 but they do, at least for certain tasks, carry three important 

 implications. Firstly, part of the variation between people of 

 different ages living at any one time will be due to the fact 

 that they were brought up under different conditions, and 

 for the present population these differences are profound. 

 Secondly, any study or testing of an individual will itself 

 affect his approach to subsequent situations so that a pro- 

 longed study cannot be wholly independent of the effects 

 of its own earlier stages. Thirdly, differing experience and 

 development serve to magnify the variations between individ- 

 uals, and may grossly affect their capacities. 



All this means that at the very least we must study groups 

 of people and compare substantial numbers in order to assess 

 age changes. We cannot base any valid inference about the 

 average capacity of older people from the exceptionally good 

 or bad performances of a few individuals or from the occasional 

 striking old men and women held up as examples of what 

 older people can do in industry, politics and other walks of life. 

 We have to think in terms of average trends, yet even these 

 may upon occasion be of doubtful significance. A common 

 finding is that the performances of older and younger people, 

 say of a group in the twenties and another in the sixties, show a 

 clear change of average performance but with a much wider 

 scatter in the older age group than in the younger — many of 

 the older group are well within the range of the younger, 

 others well outside. The problem in a case like this is to decide 

 whether the older group is comprised of some individuals who 

 have changed greatly and some who have not changed at all, 

 or whether all have changed to some extent with amount of 

 change following a non-linear function. Only in the latter 

 case can we justifiably regard the arithmetic mean as a repre- 

 sentative measure of the performance of the older group. In 

 the former case we should have to substitute for the mean a 



