An event occuring between pretest and posttest other than the independent variable that could affect the dependent variable

The best way a researcher can control for the history effect is to use a control group that was not exposed to the event

Changes occuring in participants because of passage of time

A researcher can have control over the maturation effect by using a control group

Taking a pretest somehow affects the taking of the posttest

The researcher can use a control group that does not receive any pretesting to reduce threats to internal validity

Instruments are not accurate/precise enough or do not measure what they are supposed to measure

To reduce this threat to internal validity, researchers can use well-designed instruments

Participants selected in a non-random manner differ in some way

To reduce selection bias, it is better to recruit volunteers and randomly assign them to groups instead of allowing the volunteers to self-select a group. Also, matching participants on selected characteristics and then randomly assigning them to groups can also reduce selection bias. Pretesting groups on measures of the dependent variable to make sure there are no pretreatment differences between groups is another control.

Using intact groups that vary in some element of maturity

To avoid this threat to internal validity, the researcher can pretest or prescreen groups on maturity levels

Extremely high or extremely low scores regress toward mean

To reduce this, the researcher could follow a random sample representing the full range of scores rather than placing participants in high and low groups based on one testing

Participants drop out of the study or cannot be located

Obtaining good demographic information about study participants at the beginning of the study and then determing if the group makeup has changed at the end of the study can help measure the impact of mortality on research

Altered behavior due to the effects of being studied and observed

To control for this, the researcher may 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

Altered behavior because of expectations

To control for this, the researcher can make sure the control and experimental group receive the same information so that both groups would have similar expectations.

Treatment of experimental group spills over to comparison or control groups

To control for this, the researcher can make sure that the control and experimental groups live far away from each other and have little interaction

The location of program or data collection affects participation responses

The best way to control for a location effect is to make the locations the same for all participants

Differences in persons presenting a program affect the program

To control for this, have someone else other than the program developer present the program whenever possible

External threat to validity that concerns the ability of a researcher to generalize the results of a study beyond the groups involved in the study due to the way the study groups were selected

To control for this, the researcher should be certain that the group used is an intact group, and not generalize beyond that group

This external threat concerns the extent to which the environmental conditions or setting under which an experimental study was conducted can be duplicated in other settings