© Paul Cooijmans
The variance is the mean of the squared deviations. A deviation is the difference between a value (score) and the mean of the sample. The variance is the square of the standard deviation. The variance of a variable may be visualized as a square, the length of any of its sides representing the standard deviation.
Mental test score variance may be analyzed in different ways, resulting in different ways of splitting up the variance:
- In reliability analysis, the variance is effectively split up into a true score component and an error component;
- In factor analysis, the variance is effectively split up into components accounted for by the general factor (= that which is common to all tests), by one or more group factors (common to some but not all tests), by specificity (unique to one particular test), and error;
- In heredity analysis, the variance is split up into heritability, nongenetic effects, and error.