Study that begins after a medical intervention, such as breast implant surgery, has occurred. Therefore, it looks backward in time at events complications that happened in the past. For instance, a group of women with breast implants may be identified and then asked to allow researchers to review their medical records to obtain information on complications that they had. Women might also be asked to respond to a survey or interview about whether or not they had complications with their implants. The problem with this type of study is that it assumes that if there was a problem, it would be in the medical record or that someone would remember it accurately.