According to retrospective research, this persistent pattern might endure up to 20 years. Long-term prospective studies in GAD, like those in depression, are intended to give insight into the disorder's history, nature, and results over time. Such studies are difficult to do because GAD is a condition that often goes untreated, so following its course over time is challenging. When studying GAD from first onset to recovery or relapse, it is important to control for the possible effects of medication or other interventions. This can be done by comparing people who have been treated with those who have not. The former group will show less severity at follow-up if treatment worked well; the latter group will show more severe symptoms if their conditions were not properly managed. In one long-term study of GAD, about half of participants had recovered after three years and almost all had recovered after eight years. However, nearly a third still had symptoms after 12 years. It is also important to consider how much symptom improvement is needed to classify someone as "recovered." Some researchers suggest that people be considered recovered if they no longer meet diagnostic criteria for GAD or if they do so while still maintaining normal levels of anxiety sensitivity (i.e., fear of anxiety-related symptoms). Others require a lower threshold for symptom improvement, such as cutting back on worry time by half or more.