History effect

It is one of the threats to internal validity that occurs when events take place between a pretest and a posttest of a study. The events might affect participants and impact the dependent variable.

In order to address this problem, the researcher could use a control group which is a group of participants who do not receive the treatment.

Maturation effect

It is one of the threats to internal validity that occurs when there are changes in participants due to the time that has passed between the beginning and the end of the study. The changes may not be related to any treatment effects.

In order to address this problem, the researcher could use a control group (a group with no treatment). The differences between the control group and the treatment group will show the results of the treament.

Pretest (baseline) measurement

It is a measurement administered in order to obtain the status or level of a variable before a treatment. It is compared to posttest measurement to compare the effectiveness of the treatment. The threat to internal validity occurs when participants learn how they "should" answer the questions during the posttest from the pretest.

In order to address this problem, the researcher could use a control group that does not receive a pretest.

Testing effect

It refers to the situation when differences between a pretest and a posttest might be a result of participants' learning how to answer question in the posttest from taking the pretest.

The researcher could use a control group that does not receive a pretest in order to address this concept.

Instrumentation

It is one of the threats to internal validity that takes place when measurements are not accurate, or procedures are not standardized.

In order to address this problem, the researcher should make sure that the instruments measure what they are supposed to measure. The questions on written instruments must be carefully designed. While using  physical instruments, the researcher has to make sure they are calibrated carefully. The researcher must have training in using the equipment and needs to follow the procedures strictly.

Selection bias

One of the threats to internal validity which occurs when participants are selected in nonrandom manner.

There are a few strategies a researcher can apply to address the problem, including:

• recruiting volunteers and randomly assigning them to groups
• matching participants on selected characteristics and randomly assigning them to groups
• pretesting groups on measures of the dependent variable

Selection maturation effect

A threat to internal validity that is a combination of a selection bias and a maturation. It takes place when intact groups vary in their maturation level.

A researcher may address this problem by pretesting and/or prescreening groups on maturity level.

Statistical regression

This threat to internal validity takes places when participants are selected on the basis of their extremely high or low scores.

In order to address this concern, the researcher could use a random sample of participants who represent the full range of scores.

Mortality

This threat to internal validity occurs when the researcher loses participants over the course of a study.

A researcher might try to replace the lost participants. However, this may be problematic if  the new participants differ in some important ways from the original group.

Hawthorne effect

This threat to internal validity occurs when participants' attitudes affect their behavior. It takes place when the experimental groups gets special attention over the control group. The differences in the dependent variable might be a result of that special treatment.

In order to address this problem, the researcher might try to provide the control group with similar treatment.

Placebo effect

This threat to internal validity is caused by participants' expectations not the treatment.

The researcher may address this problem by giving the control group a placebo but not letting this group know. Experimental and control groups should receive the same information about the treatment.

Diffusion of treatment

This threat to internal validity occurs when the treatment that is applied to one group affects another group.

The researcher needs to separate the experimental and control group. If this is impossible, the researcher needs to explain the nature of the study to the participants.

Location effect

This threat to internal validity occurs when there are differences in the locations where interventions take place.

The researcher might address this problem by making the location the same for all the participants. If this is not possible, the researcher should try to minimize all the differences between the locations.

Implementation

This threat to internal validity occurs when the treatment is not implemented in equal way, or one group is favored over the other one.

The researcher may control this problem by making sure all the individuals who implement the study follow standard procedures, are trained and competent. In order to avoid inequality caused by favoring one group, the researcher might have someone neutral present the treatment.

Selection treatment interaction

This is the threat to external validity that concerns the researchers' ability to generalize the results of the study. A danger of overgeneralization occurs.

If the researcher uses a random sample of participants, the results may be generalized to the study population. The researcher should not generalize to outside groups.

Setting treatment interaction

This is a threat to external validity which concerns the ability to apply the environmental conditions and settings of the study into other settings.

The researcher needs to duplicate the setting very closely while applying the treatment to a different population.

History treatment interaction

This is a threat to external validity which occurs when the researcher tries to generalize the results to past and future situations.

The researcher needs to duplicate the historical setting when applying the treatment to a different population.