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Suppose a child welfare agency keeps good records of the rate of out of home placement in cases of child abuse/neglect starting in 2000. In 2005, a new policy is created to improve family preservation services. The agency continues to keep records in the same way until 2010. In 2011, the agency wants to know if the policy decreased out of home placements. What conclusions could the agency make?
Because of the repeated assessments, the agency could look at trends before and after the policy change. The more observations someone has over a long time period, the more confidence one has in the relationship between the independent and dependent variable.