Normality and linearity of variables
Standardized skewness and kurtosis statistics were calculated; these indicated that the
distribution of some subtests deviated from a normal one (picture completion,
vocabulary, digit-symbol coding, matrix reasoning, comprehension, symbol search and
letter-number sequencing were highly skewed, and picture completion suffered from
positive kurtosis). However, given that the data were taken from a clinical sample, this
might be expected, and does not necessarily degrade the analysis (Tabachnick & Fidell,
2001). Inspection of scatterplots suggested that in some pairs of subtests there is a linear
association, while in others there is not. Again, this would be expected given the nature
of the data set, and does not invalidate the results of factor analysis.