The choice of performance measures is limited by the availability of data and the cost of collecting it. Data on wages and employment retention may require clients and their employers to provide employment information to the welfare agency frequently. Performance measures usually focus on short-term employment, because once clients leave the program it is difficult to collect data on them. Due to data limitations, job retention is often defined in terms of the client being employed a certain number of days after placement, rather than continuous employment at the same job. The quality of service provision is also particularly costly to measure, often requiring case reviews or client or employer surveys.