50 students in classes the instructor was teaching volunteered to participate in an experiment. The instructor then randomly assigned 25 of them to an experimental group and 25 of them to a control group. A police trainer is planning an experiment to test the effectiveness of a new training program which is not very popular among police officers. In order to prevent bias from entering into the experiment, the trainer decides not to give a pretest prior to starting the experiment. No metal detectors are set up at the other end of the school, called the control zone, but students are told they are hidden anyway.
Correctional researchers develop a new device to improve inmate safety.
They test the device on 50 newly admitted inmates. A pretest determines that 1 out of every 5 newly admitted inmates is injured within a 1 month period. A posttest determines that 0 out of every 5 newly admitted inmates is injured within a 1 month period. The researchers claim their device is a success. Court personnel assign every 5th defendant to the best lawyer in town, and every 10th defendant to a student intern.