Lechner and Wunsch (2013), using an unusually rich data set, find that a wide range of control variables, which are not often available for this type of study, affect impact estimates. She also warns that estimated positive impacts for short-term training within firms may partly reflect the selection of participants by firms. The Turning category in the OECD/EC Labour Market Programme Database can include longer-term job-search training, work experience with some training elements, classroom training, training within firms, and pre-apprenticeship programmes and special support for young apprentices.