Please assist me with the assigned homework by Professor Jeffrey Winter.

Internal Validity Threats - Instructions andResource Sheet

When discussing internal validity, it is often useful tohavea framework for evaluating an experiment. This activity was developed using the acronym "MRS SMITHID" based on common threats to internal validity identified by Campbell and Stanley (1966).Below is a handy review that you can use while you complete your assignment.

The potential threats to internal validity discussed in this activity:

Maturation

Regression to the mean

Selection of subjects

Mortality

Instrumentation

Testing

History

Interactions with selection (maturation, history, instrumentation)

Diffusion (imitation of treatment)

THREATS TO INTERNAL VALIDITY

"MRSMITHID" (Campbell & Stanley, 1966)

MATURATION: physiological processes occurring within the participants that could account for any changes in their behavior

Subjects may change betweentestsessions of the experiment such that any changes in scores between testing sessions may simply be due to the passage of time rather than any treatment effects.

Examples:

Aging Processes: simply growing older; changes in motor coordination; cognitive development (cf. Piaget)

Physiological States: hunger, fatigue; attention span; motivation

REGRESSION TO THE MEAN: the tendency that participants who receive extreme scores when tested, tend to have less extreme scores on subsequent retesting even in the absence of any treatment effects.

This phenomenon is the result of the fact that all measurement instruments are not perfectly reliable (i.e., there is measurement error present). It is this error that most likely accounts for the extreme score, not some inherent characteristic within the individual.

As a result, a person's score tends to fluctuate on repeated testing. Extreme scores typically become less extreme. The implication is that the difference between groups formed based on extreme scores tend to become smaller even in the absence of any treatment effects.

EXAMPLE: Notice how the difference between the top two scores and the bottom two scores decreased with the second testing. What if a researcher wanted to see if a treatment program would reduce the discrepancy between the top and bottom scorers? The regression to the mean effect suggests that this discrepancy will become smaller even if the treatment was completely ineffective.

Subject Test score

Re-test score

Al

99

94

Bob

90

86

Carl

75

77

Dan

62

70

Ed

32

52

SELECTION OF SUBJECTS: Any bias in selecting and assigning participants to groups that results in systematic differences between the participants in each group.

The differences exist before one group is exposed to the experimental treatment.

This threat to validity is great in quasi-experiments where the random assignment to treatment conditions is not possible.

MORTALITY: Differential dropping out of some subjects from the comparison groups before the experiment is finished, resulting in differences between the groups that may be unrelated to the treatment effects.

The problem is that the subjects who drop out of the study for whatever reasons may be different than those who complete it. This may inflate, obscure, or confuse the treatment effects of interest.

The researcher excluding the data of particular subjects based on some criterion can cause this bias.

INSTRUMENTATION: Changes in the measurement procedures may result in differences between the comparison groups that are confused with the treatment effects.

For example:

Observers may become more experienced or careless over time which results in differences between the pretest and posttest measurements that are unrelated to the treatment effects.

Calibration of testing apparatus may change from one test to another.

A change to a "better way of collecting the data" between the pretest and the posttest such as finding better ways to ask the same question of the participants.

TESTING: When participants are repeatedly tested, changes in test scores may be more due to practice or knowledge about the test procedure gained from earlier experiences rather than any treatment effects

Similar to maturation except that the change is caused by the testing procedure itself.

HISTORY: Extraneous events occurring during the course of the experiment that may affect theparticipants' responses on the dependent measure.

Could be major events occurring in society (e.g., social upheaval) or minor events occurring within theexperimental situation (e.g., equipment malfunction)

These events may account for the participants' responses in the experiment more so than thetreatment of interest.

INTERACTION: SELECTION BY MATURATION, INSTRUMENTATION, or HISTORY: The treatment and no-treatment groups, although similar at one point, would have grown apart (developed differently) even if no treatment had been administered.

Even though pretest scores may have been the same, groups that are not matched as well on other relevant variables that may cause the groups to naturally become different after a period of time

EXAMPLE: Long-term Head Start research comparing middle-class and disadvantaged children.

DIFFUSION (or imitation of treatment): Threat to internal validity that can occur if participants in one treatment group become familiar with the treatment of another group and copy that treatment.

EXAMPLE:Studentsexposed to a new teaching method in a 9:00 class may share their insights with a 10:00 class not exposed to those methods. Both might start to succeed in their studies,thusno differenceswillemerge since both groups are using the same method