When Jessica sits down to do her homework and study, she is easily distracted by the television in another room. Jessica demonstrates a deficit in ____

Why do researchers use random assignment to place participants into one of two or more groups?

a. To eliminate the effect of the independent variable on the dependent variable. b. To randomly distribute the presence of potentially influential differences between groups and increase the chance of drawing a causal inference. c. To make sure that groups in a study are as different as possible at the beginning of the study. d. To correlate the independent and dependent variables.

Tim's parents brought him to a psychological clinic when he was 5 years old. The clinician's

assessment shows that he currently meets diagnostic criteria for severe mental retardation and autism. However, Tim's parents report that he was a "normal little boy" until 6 months ago. Then, over a period of a few weeks, many aspects of his behavior deteriorated very rapidly. Which of the following diagnosis best fits this description of Tim's symptoms: a. Autism b. Asperger's disorder c. Rett's syndrome d. Childhood disintegrative disorder e. Childhood-onset schizophrenia