MECHANICAL ABILITY 307 
(3) It was definitely established that the group- -factor in the * mechanical ’ 
group was not the same as the group-factor in the routine ‘ manual ’ tests. 
The Routine Manual Factor.—Statistical analysis of the manual tests 
along similar lines indicated that : 
(1) The specific intercorrelations of the routine ‘manual’ assembling 
and stripping tests, and the simple manual tests, could be best explained 
by a single group-factor. 
(2) This routine (or ‘ manual’) factor was clearly distinguishable from 
the ‘mechanical ’ factor seen in the mechanical aptitude and mechanical 
assembling tests. 
(3) In general, the more complex assembling tests were more highly 
saturated with this factor than were the simpler manual tests. 
(4) Where the tests were both very simple and very similar (such as 
screwing and unscrewing the turnbuckle), small additional factors common 
to the pair of tests concerned, and to these only, were observed. 
The ‘ Abilities’ in Assembling Work.—The independently measurable 
‘ abilities ’ or ‘ group-factors’ in assembling work were thus found to be 
(i) amechanical factor, associated with the solution of a mechanical problem ; 
(ii) a routine manual factor associated with the manual activity involved 
in this work ; and (iii), to a less extent, general intelligence. As the work 
assumes a routine character the mechanical factor tends to disappear. 
There was little evidence of the routine factor in the mechanical assembling 
operations. In these the manual activity involved appears to function 
specifically, rather than as a group-factor. 
The Organisation of Manual Activity.—The more complex forms of 
manual activity appear to depend on a broader and more important group- 
factor than earlier work on simpler manual tests would lead us to suppose. 
As the operations become simpler they depend less upon this common 
factor and more upon factors specific to the particular operation. ‘The 
measurement of this group-factor, in relative independence of other factors, 
as provided by suitably constructed tests, would seem to be essential 
wherever vocational guidance or sélection in the sphere of manual activity 
is in question. 
III. THe MeEntTat Processes IN MANUAL ACTIVITY. 
The analysis divides into two parts. The first attempts to elucidate the 
cognitive processes involved in the solution of the mechanical problem 
which accompanies certain forms of manual activity such as that of the 
mechanical assembling operations. It thereby extends to manual activities 
the analysis of mechanical aptitude which the writer has already described 
in a former work.. In the former analysis the problems were of a different 
kind and were uncomplicated by manual activity. The present extension 
of the analysis to include manipulative operations throws light on another 
large class of engineering occupations. 
The analysis also includes an examination of the processes underlying 
the cognition of shape, and the relation of these to drawing and design. 
The results are therefore of vocational interest wherever the worker is 
called upon to deal with spatial material. 
_ The second part of the analysis deals with those manual activities which 
involve no special mechanical problem and which we termed routine 
assembling operations. It attempts to unravel the cognitive processes 
associated with the manual factor which our objective measurements 
disclosed. It includes an account of the kind of knowledge that is acquired 
