Threatens validity when events occur between the pretest and post-test of a research study that could affect participants in such a way to impacts the dependent variable.

The process of maturing which takes place in the individual during the duration of the experiment which is not a result of specific events but of simply growing older, growing more tired, or similar changes. 

The pretest or baseline measurement is usually compared to a posttest measurement to determine the effectiveness of a program. Effect is determined by any differences seen from a baseline to a posttest.

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

The actual physical instrument that collects data in an experiment. Written instruments must accurately measure what they are suppose to or internal validity will be effected.

Researchers must use well-designed instruments reduce this threat to internal validity.

When individuals are selected in a nonrandom manner, which may reduce internal validity. Volunteers must often be recruited to participate in studies because it is not possible to use random selection and require those selected to participate in a treatment.

Occurs when using intact groups that vary in their maturation level.

Groups are chosen because of extreme scores of measurements; those scores or measurements tend to move toward the mean with repeated measurements even without an experimental variable.

participants drop out of the study or cannot be located. Loss of subjects can create problems in terms of group comparisons when the number of participants in some groups fall below established guidelines for statistical analysis.

Altered behavior due to the effects of being studied and observed. Anytime an experimental group receives special attention over the control group, any noted differences in the two groups can be a result of special attention given in the experimental group and not effect the independent group.

Altered behavior because of expectations rather than treatment. A control group usually receives a placebo in order to measure the differences between the control and experimental group.

Occurs when the treatment being applied to one group spills over or contaminates another group.

Can be difficult in both the control and experimental interact. However, researchers can explain  the importance of keeping some aspects confidential or they can find a different community.

Individual(s) responsible for implementing the experimental treatment and the possibility that they may inadvertently introduce inequality or bias into the study.

Concerns the ability of a researcher to generalize the results of a study beyond the groups involved in the study.

Use of random sample of participants, results can be legitimately be generalized to the group from which the random sample was selected.

Concerns the extent to which the environmental conditions or setting under which an experimental study was conducted can be duplicated in other settings.

Internal Validity  

a study that does a good job controlling for the possible effects of extraneous variables on the dependent variable.

Observed effects of the independent variable on the dependent variable are real and not caused.

concerned with the ability to generalize the study results to other groups and settings beyond those in the current experiment.

Research that can be generalized will have more value

A control group can be created to eliminate this threat.

Make locations the same for all participants. If not possible, try to minimize location differences that could impact the dependent variable.