Aresearcher examined the effect of lighting on learning. randomly assigned participants learned material while using a 100 watt or a 125 watt bulb. there were no differences between the two groups. the nonsignificant results may have occurred because of:

a. a costly measure of learning.
b. insufficient strength of the manipulation.
c. selection differences.
d. all of these

Pj conducts an experimental study on the effects of soft music during high stakes science testing. he randomly assigns students at the school. in one condition he does not provide music for testing while in the other group he does provide the music. he administers a pretest at the beginning of the year and a posttest at the end of the year. pj’s design is best described as a: a. static group comparisonb. pretest-posttest control group designc. control-group-time-series designd. nonrandomized control-group pretest-posttest design