This article deals with probable causes for missing values when using the Revised Class Play instrument for peer assessment of social behavior. The study was conducted with 10 – 13 year old primary school children in Sweden. The findings reveal that missing values may be caused
by the cognitive and linguistic complexity of the instrument (RCP), as well as by gender stereotypes. The results indicate that investigating the causes of missing values provides valuable information about the validity of the RCP. This information can be used to improve the instructions
on how and when to use the RCP.