An excerpt from BPS Reasearch Digest Blog:

The tactic used by most researchers is to recruit from the wider community, for example by advertising in the local paper. But Thurston’s team argue a large proportion of the general community actually have their own mental health problems, and many of them are receiving therapy, something many researchers fail to screen for. This means that what research papers describe as a “non-clinical community sample” may not be so “non-clinical” after all.

Thurston and her colleagues assessed 224 families recruited through adverts in local newspapers in south eastern USA as part of a larger study. They found 11 per cent of the teenagers, 20 per cent of the mothers and 13 per cent of the fathers met the diagnostic criteria for one or more psychiatric disorders. Moreover, 12 per cent of the teenagers, 20 per cent of the mothers and 11 per cent of the fathers were currently in therapy. These two groups didn’t completely overlap – for instance, there were 25 mothers who met diagnostic criteria for a psychiatric disorder but who weren’t in therapy.

Thurston’s team said their findings have implications for research validity. Differences previously identified between clinical and so-called “non-clinical” groups may be caused by a factor other than the clinical status of the two groups.

Researchers should screen their community participants to find out if they are currently experiencing mental distress or participating in therapy, Thurston’s team advised. But as regards whether such participants should then be excluded from research, Thurston and her colleagues said: “There is no perfect answer, but rather, researchers must weigh the costs and benefits of their exclusionary criteria in relation to the goals of the study.”

Thurston, I.B., Curley, J., Fields, S., Kamboukos, D., Rojas, A., Phares, V. (2008). How nonclinical are community samples?. Journal of Community Psychology, 36(4), 411-420. DOI: 10.1002/jcop.20223