an experimental design in which assignment of participants to an experimental group or to a control group cannot be made at random for either practical or ethical reasons; this is usually the case in field research. Assignment of participants to conditions is usually based on self-selection (e.g., employees who have chosen to work at a particular plant) or selection by an administrator (e.g., children are assigned to particular classrooms by a superintendent of schools). Such designs introduce a set of assumptions or threats to internal validity that must be acknowledged by the researcher when interpreting study findings. A study using this design is called a quasi-experiment. Examples include studies that investigate the responses of large groups to natural disasters or widespread changes in social policy.