Controls non experimental variables that affect the dependent variable. Selective Control - Manipulate indirectly by selecting in or out variables that cannot be controlled. Internal Validity History - The events occurring between the first and second measurements in addition to the experimental variable which might affect the measurement. During the sale a hurricane occurs and results of the study may be affected because of the hurricane, not the sale. On the Posttest they change to answers they feel are more acceptable.
Experimental group learns from the pretest.
Measuring Instruments - Changes in instruments, calibration of instruments, observers, or scorers may cause changes in the measurements. Managers who are performing poorly are selected for training. Their average Posttest scores will be higher than their Pretest scores because of statistical regression, even if no training were given. Differential Selection - Different individuals or groups would have different previous knowledge or ability which would affect the final measurement if not taken into account. A group of subjects who have viewed a TV program is compared with a group which has not.