Hello-goodbye effect: The phenomenon initially described in psychotherapy research, but one which might arise whenever a subject is assessed on two occasions, with some intervention between visits. Before the intervention a person might present himself/herself in as bad a light as possible, thereby hoping to qualify for the treatment, and impressing staff with seriousness of his/her problems. At the end of the study the person might want to 'please' the staff with his/her improvement, and so might minimize any problems. The result is to show that there has been some progress when none has occurred, or to magnify the effects that did happen.