A researcher measures mood for a group of participants who listened to happy music for 20 minutes, and a second group who listened to sad music for 20 minutes. If different mood scores are obtained for the two groups, the researcher would like to conclude that music influences mood. However, the happy music group was tested in a room painted yellow and the sad music group was tested in a room painted dark brown. The researcher is concerned the room color may have accidentally influenced mood. What kind of validity is being questioned