ABSTRACT
Quasi-experiments lack some essential features of the true experiment. They occur, for instance, when participants are not randomly allocated by the experimenter into conditions of the manipulated independent variable (a non-equivalent groups design) or where the researcher does not control the independent variable. A common case of the latter is a natural experiment. Some natural experiments can be analysed as a time-series design. The laboratory is used for the scientific purpose of isolating variables of interest to theory and controlling extraneous variables. The psychology laboratory does not usually contain the stereotypical lab equipment of retort stands and bubbling bottles. Laidlaw et al. asked volunteer participants for a psychology experiment to sit in a waiting room and to await the return of the experimenter at which point the experiment would begin.
