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Internal & External Validity
Definitions of internal & external validity and the multiple threats to both.
18
Education
11/05/2010

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Term
External Validity
Definition

Study results can be generalized to other groups and settings beyond those in the experiment.

 

Results with external validity can be of more use to the profession.

Term
Internal Validity
Definition

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

 

A well-control lab study is best for achieving internal validity.

Term
History
Definition

A threat to internal validity.


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

 

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

Term
Maturation
Definition

Threat to internal validity

 

Changes occurring in participants because of passage of time (e.g., physical, mental, etc.)

 

A control group is best to account for maturation's threat to internal validity.

Term
Testing
Definition

Threat to internal validity.

 

Taking a pretest somehow affects the taking of the posttest.

 

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

Term
Instrumentation
Definition

Threat to internal validity

 

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

 

A well-designed instrument will reduce this threat.

Term
Selection Bias
Definition

Threat to internal validity

 

participants selected in a non-random manner differ in some way.

 

There are several methods to reduce this threat:

  • Recruiting volunteers and then randomly assigning them to groups is better than allowing them to self-select.
  • Matching participants on selected characteristics and then randomly assigning them to groups.
  • Pretesting groups on measure of the dependent variable to make sure there are no pretreatment differences between groups.

Term
Selection Maturation
Definition

Threat to internal validity

 

Using intact groups that vary in some element of maturity.  A combination of selection bias and maturation.

 

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

Term
Statistical Regression
Definition

Threat to internal validity.

 

Extremely high or extremely low scores regress toward mean.

 

A study can be designed to follow a random sample rather than those with the highest or lowest scores.

Term
Mortality/Attrition
Definition

Threat to internal validity

 

Participants drop out of the study or cannot be located.  This threat cannot be completely eliminated.

 

  • Oversampling and using large group sizes is one way to overcome this threat.
  • Using incentives to encourage participants to stay in the study may be helpful.

Term
Hawthorne
Definition

Threat to internal validity

 

Altered behavior due to the effects of being studied and observed.

 

  • To control for this 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.
  • Another way of combating this is to prevent participants from knowing they are in a study.  This is difficult due to full disclosure requirements.

Term
Placebo
Definition

Threat to internal validity

 

Altered behavior because of expectations.

 

The researcher could try to make sure those in both the control and experimental groups receive the same information so that both groups would have similar expectations or to give both groups as little information as possible.

Term
Diffusion
Definition

Threat to internal validity

 

Treatment of experimental group spills over to comparison or control groups.

 

It is very difficult to control for diffusion, but it could be helpful to use a control group that does not have contact with the experimental group.

Term
Location
Definition

Threat to internal validity

 

The location of program or data collection affects participant responses.

 

Make the locations the same for all participants.  If that is not possible, minimization of location differences should be implemented.

Term
Implementation
Definition

Threat to internal validity

 

Differences in persons presenting a program affect the program.

 

Someone other than the program developer should present the program.  This person should not have a vested interest in the success nor the failure of the program.

Term
Selection Treatment Interaction
Definition

Threat to external validity.

 

Whether the researcher is able to generalize the results of a study beyond the groups involved in the study.

 

Using larger groups sizes and a random sample of participants allows for more accurate generalization.

Term
Setting Treatment Interaction
Definition

Threat to external validity

 

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

 

It is best to attempt to duplicate as closely as possible the original setting.

Term
History Treatment Interaction
Definition

Threat to external validity

 

This develops when the researcher tries to generalize findings to past and future situations.

 

The time an experiment is performed should be considered when generalizing findings.