Charles McArthur

The contagious Poisson distribution of Rorschach scores

Abstract Rorschach scores show frequency distributions that are neither normal nor Poisson. Their generating equation may be a formula for a “contagious Poisson” distribution developed by Coleman for use in social psychology. Data from the Harvard Student Study and from a sample of residents and interns both show good fit to the Coleman formula for nearly all Rorschach variables. Scores associated with abnormal mental states do not show good fit, however. The mathematical model may represent contagion once a type of response has occurred, or individual differences in response proneness, or perhaps an emission process. The corresponding urn model is one in which when one black ball is drawn, two are replaced.