Determine whether the following study described is observational or an experiment. If the study is an experiment, identify the control and treatment groups, and discuss whether single- or double-blinding is necessary. If the study is observational, state whether it is a retrospective study, and if so, identify the cases and controls.

A study of 2501 runners looked for relationships between specific types of running injuries and variables such as height, weight, body mass index, age, and running history.