Changes in Human Performance with Age 163 



problem task not only correct achievement but also time 

 taken and possibly scores indicating the manner in which the 

 achievement was attained need to be taken. 



(iv) The most radical method of overcoming the problem 

 of comparability is to abandon the use of human subjects in 

 favour of animals whose background and experience can be 

 rigidly controlled. There is, of course, some hazard in using 

 the results of animal experiments as an aid to the inter- 

 pretation of human performance, but insofar as ageing is due 

 to biological processes common to different species, animal 

 studies would seem capable of supplying invaluable checks 

 upon human research. We may note that experiments on 

 animal behaviour in relation to age seem to show very much 

 the same trends as human studies, and thus tend to confirm 

 that environmental factors cannot by any means wholly 

 account for changes of human performance with age. 



Longitudinal Studies 



The difficulties inherent in cross-sectional studies can be 

 avoided by following a group of individuals over a substantial 

 period of time. The longitudinal method does however have 

 two difficulties of its own which severely limit its usefulness. 



(a) The method essentially involves testing a subject's 

 performance and retesting it once or more later in life. This 

 requirement means that any age effects are inextricably 

 mixed with any learning effects. These latter can be very 

 marked. For example, Heim and Wallace (1949, 1950) have 

 shown that substantial gains in score on an intelligence test 

 are made when it is taken more than once even when the 

 subjects have no knowledge of their success or failure on 

 earlier occasions, and that these gains transfer to another 

 intelligence test. Their results were, however, for tests given 

 only one week apart and thus may not be an entirely valid 

 objection to longitudinal studies using intelligence tests such 

 as that of Owens (1953). The effects of a single experience can, 

 however, without doubt be long lasting and we need to know 

 more about them before we can be confident as to the meaning 



