These papers from what I was able to find, lay out the basis of the Wisconsin Card Sorting Test (WCST). There are three possible rules for sorting the cards; color, shape, or count. I am curious why the WCST places 4 stimulus cards in front of the patient when 3 carefully chosen cards would be sufficient. With 4 cards it seems that unless the examiner makes special preparations, there might be occasions when there is more than 1 correct stimulus card to choose, or during a transition from certain rule to the next the patient may make an incorrect guess and won't be able to tell which rule to eliminate.
Can anyone explain why there has to be 4 stimulus cards? Could I run an equally effective WCST with 3 stimulus cards?
Grant, D. A., & Berg, E. (1948). A behavioral analysis of degree of reinforcement and ease of shifting to new responses in a Weigl-type card-sorting problem. Journal of experimental psychology, 38(4), 404.
Fey, E. T. (1951). The performance of young schizophrenics and young normals on the Wisconsin Card Sorting Test. Journal of consulting psychology, 15(4), 311.