The estimated population variance of the scores 2, 4, 6 is _____

Bernadette, a participant in a longitudinal study, became quite familiar with the test over time and, as a result, her performance improved. This limitation of longitudinal research is known as

A) biased sampling. B) practice effects. C) random assignment. D) cohort effects.

Which of the following could you learn to do in your sleep?

a. math b. foreign language c. touch a switch on cue d. dream analysis