ON VOCATIONAL TESTS. 273 



• reliability ' — the coefficient differing little from day to day during the practice 

 period. 



II. Relating to accuracy of measurement in intelligent assembling. • 



(a) The average inter-correlation of the three ' intelligent ' assembling tests is 

 .33 for the boys and .38 for the girls. 



(b) The two tests of ' mechanical aptitude ' correlate .72 (boys) and .59 (girls) 

 with one another, and yield correlation coefficients of similar magnitude when com- 

 pared with themselves by determining the correlation between the pool of 'odd' 

 sub-tests with that of 'even' sub-tests. It is shown later that these involve the 

 same ' mechanical ' factor as that in the intelligent assembling tests. 



III. Relating to Influence of Practice on Ability. 



(a) The general influence of practice is to draw individuals closer together with 

 respect to ' ability. ' 



(b) There is a well-marked tendency for individuals to inaintain, during practice, 

 the rank order with which they begin. 



(c) Generally speaking, those weaker at assembling (routine) exhibit greater 

 variability from daj' to day, i.e. their practice, curves are less smooth. 



(d) They also eflect more improvement during the period of practice, whether 

 this be measured ' absolutely ' or in relation to their ' ability.' 



(e) There is a small positive correlation between ' general intelligence ' and 

 ' ability. ' 



(/) The correlation between ' general intelligence ' and ' improvability ' is, if anything, 

 -negative. This does not mean that, given equal initial ability, those who are less 

 intelligent will tend, on this account, to improve more with practice, but that those 

 who initially rank low at assembling have an easier task to efiect a given amount of 

 improvement. 



(g) It follows from the foregoing observations that the scores made initially at a routine 

 assembling test provide a better criterion as to the ability to which an individual may 

 subsequently attain, through practice, than do either his ' general inteUigence ' or his 

 ' improvability.^ 



IV. Relating to the Factors involved in ' InUlligent Assembling.' 



(a) The various tests of 'intelligent ' assembling correlate (i) with one another, 

 (ii) with tests of ' mechanical aptitude ' and (iii) with ' general intelhgence. ' 



(6) The application of Spearman's ' tetrad-difference ' criterion to these inter- 

 correlations denotes the presence of (i) a small factor running through all of these 

 tests, and (ii) a group factor common to the ' intelligent ' assembling and ' mechanical 

 aptitude ' tests. 



(c) The inter-correlations indicate no new 'mechanical' factor peculiar to the 

 (practical) ' intelligent ' assembling tests, over and above the 'mechanical ' factor in 

 the ' mechanical aptitude ' tests. 



(d) The best tests of this mechanical factor are the more difficult assembling 

 tests and the mechanical aptitude tests. The simplest assembling tests provide little, 

 if any, measure of the mechanical factor. This is probably due to the facts (i) that in 

 such simple tests the scores of those who possess good mechanical ability is almost 

 wholly determined by the time taken to effect the practical (motor) work of assembling 

 rather than that taken to think out how to assemble, and (ii) that such thinking out 

 as occurs is based on previous experience rather than on original thought. 



V. Relating to the factors involved in 'Routine Assembling.' 



(a) There is a general positive correlation running through all the routine 

 assembling tests and the tests of general intelligence. 



(b) The application of the ' tetrad-difference ' criterion indicates the presence of a 

 group factor common to the routine assembling tests. 



(c) The more difficult motor processes involved in ' assembling ' appear more 

 highly saturated with this factor than do the easier ' stripping ' processes. 



VI. Relating to Transfer of Practice Effects. 



(a) There is little evidence of transfer of practice effect from one routine operation 

 to another. 



(b) The mere acquisition of speed in effecting a series of movements; as involved 

 in this practice, must, however, be distinguished from the kind of ' training ' which 

 might be given by a competent psychologist, after having practised the operations 

 himself. Such training would involve — (i) the determination of the best method of 



1930 



