Consider the following scenario:
60 children, ages 3-5, are selected to participate in a program for developing non-verbal communication skills. The children were selected on the basis of extremely low scores on a test of non-verbal communication. After participating in the program, the children are re-tested. An average improvement of 59% is observed in the quality of non-verbal communications.
What threat to internal validity is present in this situation? How can this confound be eliminated?