A History Effect threatens internal validity when events occur between the pretest and posttest of a research study that could affect participants in such a way as to impact the dependent variable.

The best way to control for the history effect is to use a control group that is not exposed to such events.

A maturation effect occurs when changes are seen in subjects because of the time that has elapsed since the study began and that may not be the result of any program effects.

A control group can eliminate this threat to internal validity.

A testing effect occurs when participants learn from the pretest and are "test wise" for the posttest.

Use of a control group that does not receive any pretest will reduce this threat to internal validity.

Selection Bias may be present when individuals are selected in a nonrandom manner.

The threat of selection bias can be somewhat controlled by recruiting volunteers then randomly assigning them to groups, by matching participants on selected characteristics than randomly assigning them to groups, and by pretesting groups on measures of the dependent variable to make sure there are no pretreatment differences between groups.

A Selection Maturation Effect occurs when using intact groups that vary in their maturation level.

Pretesting and/or prescreening groups on maturity levels is a way to avoid this threat to internal validity.

Statistical Regression concerns occur when participants are selected on the basis of their extremely high or low scores.

To help with this concern to internal validity, random samples representing the full range of scores could be used.

The Mortality/Attrition effect happens when participants die, can no longer participate, or move away without leaving contact information.

Obtaining good demographic information before the study can help, as well as determining if the group makeup has changed at the end of the study.

A Hawthorne Effect occurs when participants' attitudes toward being involved in a study affect the way they behave.

To control for the Hawthorne Effect, a researcher may try to provide the control group with some type of special treatment that is comparable to the experimental group but would not have a direct impact on the dependent variable.

A Placebo Effect is caused by participants' expectations rather than by any provided treatment.

To control for a placebo effect, the researcher tries to make sure that those in both the control and experimental groups receive the same information so that both groups would have similar expectations.

A Diffusion Effect occurs when the treatment being applied to one group spills over or contaminates another group.

Controlling for a diffusion effect would require using a control group that has no communication with the experimental group.

A Location Effect occurs when there are differences in the locations where interventions take place.

The researcher should try to do everything possible to minimize location differences that could impact the dependent variable.

An Implementation Effect involves the individual or individuals responsible for implementing the experimental treatment and the possibility that they may inadvertently introduce inequality or bias into the study.

To control for this effect, the researcher should have someone other than the program developer present the program.  The person should be neutral and not have a vested interest in the outcome of the study.

The Selection Treatment Interaction concerns the ability of a researcher to generalize the results of a study beyond the groups involved in the study.

Results should only be generalized to the population who could have been chosen randomly to participate.

The Setting Treatment Interaction concerns the extent to which the environmental conditions or setting under which an experimental study was conducted can be duplicated in other settings.

The researcher should be try to make sure the environmental conditions do not effect the results of the study or could be duplicated.

The History Treatment Interaction develops when the researcher tries to generalize findings to past and future situations.

The researcher should not assume that results will be the same in the future as they were in the past.