A high rate of failure on effort tests by undergraduate students volunteering for Psychology experiments raises questions about the validity of such studies if effort is not measured. 

http://acn.oxfordjournals.org/content/27/8/849.full

As far as I am aware this is the first study in which effort testing has been appplied to undergaduates volunteering for research studies to get course credits. It seems that many are not highly motivated to try their best and they produce invalid test results. The authors conclude that such studies need to employ effort tests.

Comment: The only sample in which all cases pass effort tests will be those in which people have a very high motivation to do well. One example of such a group would be police applicants. In over 30 consecutive cases tested by Dr. Paul Green, no case failed any effort test or even came close to failing one. The tests included the WMT, MSVT, NV-MSVT, CVLT Recognition Hits and Reliable Digit Span.