Wendy wants to study how easily people make friends. She assigns people to talk with a stranger for either five minutes or ten minutes, and assigns people who are more outgoing to the "five-minute" condition, and shy people to the "ten-minute" condition. Which of the following is the threat to the internal validity in this study?

Answer: Wendy did not use random assignment

Explanation: Wendy's failure to randomly assign participants or subject to his research will pose a threat to the internal validity of his study. The internal validity of a study simply refers to the structure of an experiment and it's trustworthiness in establishing causal relationship between the applied treatment used a study and the observed outcome. Wendy's decision to ignore random assignment and opt for a manual assignment of subject to each treatment will pose a threat to the structure of his study and hence the establishment of a causal relationship between the treatment t and outcome of the study.