27 Dr. Feliciano is conducting a study of sleep on children's activity level that takes place over several days; however, Halloween falls in the middle of his study, and he notices that on Hallowee...

27

Dr. Feliciano is conducting a study of sleep on children's activity level that takes place over several days; however, Halloween falls in the middle of his study, and he notices that on Hallowee children are especially active during his observation. Which threat to internal validity has this study?

a. Subject selection bias

b. Volunteerism

c. Fatigue

d. History