The purpose of the pre-test is to do a check on an ad or commercial  before it is launched in the media or is being further developed. As  stimulus material a storyboard can be used or some rough sketches.  Sometimes only some thought are available that are described in a few  lines. By using individual in-depth interviews the target group can  react to the concepts or ideas. The interviews are very open and first  impression, message and evaluation of the concept are discussed in a way that is strongly directed by the respondent. The goal is to make small  adjustments to the ad or commercial if needed. Experience learns that  only minor adjustments can lead to a major change in the perception of  the ad or commercial.
The pre-test is also a simple way to check  wether some issues are overlooked: is the perception of the service or  the product different than we thought? Are we missing something? For  this reason it is also described as a ‘disaster-check’.
Important is to describe beforehand what communication target the ad or commercial have.