A researcher wants to examine the effectiveness of a new educational program with as little disruption to current class procedures as possible. Therefore, she assigns all of the students in Ms. Paul's class to the treatment group and all of the students in Ms. Spencer's class to the comparison group. Afterwards, the researcher wonders whether the difference between the groups on the posttest could be due to the different initial make-up of the two classes. This researcher is wondering about which threat to validity?

Group of answer choices

Statistical power

Selection

Attrition or mortality

History