Internal Validity

• Summary:  The extent to which the observed effects of the independent variable on the dependent variable are real and not caused by extraneous factors.

• Address the concern:  The more tightly controlled the effects of extraneous variables on the dependent variable the more likely to have a strong internal validity. 

External Validity

• Summary:  Ability to generalize the study results to othe rgorups and settings beyond those in the current experiment.

• Address the concern:  The more tightly controlled a study, the less likely to have external validity.

History Effect

• Summary:  Threatens internal validity when events occur between the pretest and posttest that could affect participants in a way to impact the dependent variable.

• Address the concern:  Use a control group that is not exposed to the intervention. 

Maturation Effect

• Summary:  factors that may influence subjects' performance because of the time that has elapsed. 

• Address the concern:  Include a control group in the research design. 

Testing Effect

• Summary:  Any differences seen from baseline to posttest. 

• Address the concern:  Use a comparison group that is exposed to the dependent variable but does not take pretest and increase length of of time between administration of pretest and posttest. 

Instrumentation

• Summary:  any changes in the instrument or measuring device used to test the effect of the dependent variable.  

• Address the concern:  Assure the posttest and pretest are the same and procedures for recording data remain the same each time data is collected.   

Selection Bias

• Summary:  selection of participants in a nonrandom manner to reduce internal validity. 

• Address the concern:  Recruit volunteers and then assign them to groups randomly.  Also match participants on selected characteristics and then randomly assign them to groups.  Also pretest groups on measures of the dependent variable to make sure there are no pretreatment differences between groups. 

Selection Maturation Effect

• Summary:  A threat to internal validity that combines a selection bias with a maturation threat.

• Address the concern:  Pretesting and/or prescreening groups on maturity levels.

Statistical Regression

• Summary:  Occurs when participants are selected on the basis of their extremely high or low scores. 

• Address the concern:  Assure that the groups are composed of subjects whose scores represent the full range of possible scores. 

Mortality/Attrition Effect

• Summary:  Loss of subjects in an experiment.    

• Address the concern:  Obtain good demographic information about study participants at the beginning and then determine if the group makeup has changed at the end of the study after mortality has occured.  

Hawthorne Effect

• Summary:  Occurs when participants' attitudes toward being involved in a study affect the way they behave.

• Address the concern: 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.  Also, keep participants from knowing that they are apart of the study.      

Placebo Effect

• Summary:  Caused by participants' expectations rantehr than by any provided treatment. 

• Address the concern:  Assure that those in both the control and experimental groups receive the same information so that both grouups would have similar expectations.  Also, give both groups as little information as possible so as to reduce any expectations. 

Diffusion Effect

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

Address the concern:  Utilize a different control group in another location and match the groups according to age, race, gender, health condition, and so forth.  If one group is involved, make sure to explain the nature of the study to participants, emphasizing the need to keep all in-chair exercises confidential until end of study period. 

Location Effect

• Summary:  Occurs when there are differences in the locations where interventions take place.

• Address the concern:  Make the locations the same for all participants.  Also, do everything possible to minimize location differences that could impact the dependent variable. 

Implementation Effect

• Summary: Involves the individual or individuals responsible for implementing the experimental treatment and the possibility that they may inadvertantly introduce inequality or bias in the study.   

• Address the concern:  Have a neutral person who is not vested in the study present the program.  Another idea is to have a neutral observer watch the presenter with specific instructions to look for ways in which the two groups are being treated differently.     

Selection Treatment Interaction

• Summary:  the abiliity to generalize the results of a study beyond the groups involved in the study. 

• Address the concern:  Have a difference in both the participants and the setting.

History Treatment Interaction

• Summary:  Threat to external validity that develops when the researcher tries to generalize findings to past and future situations.

• Address the concern:  Consider what groups one can legitimately generalize to, and do not generalize to other outside groups.