Chapter 9

Internal Validity Threats to Internal Validity Alternative explanations Alternative hypotheses The DV is NOT dependent upon the IV but instead is dependent on something else

Subject characteristics –Age –Gender –Grade in school –Intelligence –SES

Loss of subjects Mortality (death) Drop out of school Move out of town Stop participating

Location Environmental factors –Factors related to the setting Lighting Noise Changes in setting

Instrumentation Instrument decay –The instrument changes over time grading papers, fatigue of teacher Battery grows weaker –Data collector characteristics Gender Authority figure –Data collector bias Different administrators of test Inadvertent body language Training can overcome this

Testing effect Being exposed to the testing has an effect –Pre-test informs participants –Test-retest, first exposure provides practice

History Events that occur to the participants during the span of the data collection In experimental study, one group has a different experience than another group

Maturation The participants change over time – over the span of the data collection –Institute a character education program for 8 th graders. Over the year’s time, the 8 th graders experience perceive themselves as more able to stand up for themselves

Attitude of Participants Hawthorne Effect In experimental study, one group finds out the nature of the other group

Regression to the Mean The Pre-test differs from the post-test, score moves to the overall average No where to go but up (down).

Implementation Usually shows up with experimental groups Researcher may favor one method over another and subtly sets up one group for better performance The difference between the selection of two groups is not random but an unrelated factor

The pre test allows or cues the participants into learning the nature of the study.

Two existing groups are compared with respect to their scores on an achievement test.

A car accident occurs outside the school during an algebra test for one class, but another class completes it un-interrupted.

An art program is instituted at two grade levels, 2 nd grade and 7 th grade. The 7 th graders show a much greater change in their skills than the 2 nd graders. Conclusion is that the program is more appropriate for 7 th graders.

High school students in private schools score better on an instrument designed to test for financial awareness than students in a public high school.

The scorers of an exam unconsciously grade the exam papers for a comparison group in a way the some students receive higher grades than another group of students.