There are also some notably weaknesses of retrospective school surveys. School records may be incomplete and this bias can be differential by school and potentially related to demographic factors. If parents complete these forms (6 states in US), the forms may suffer from information bias. Children who migrated out of the study area before school entry and children who are home schooled are typically not included in these studies. Additionally, the data may be several years old and therefore of less utility than studies that collect more current data.