Lecture 3C: Internal Validity Flashcards
What is internal validity?
The degree to which the relationship between the independent and dependent variables is free from the effects of extraneous (confounding) factors.
Portney & Watkins 2009, p. 869.
Define confounding variables.
Any factor that is not directly related to the purpose of the study, but that may affect the dependent variable.
Portney & Watkins 2009, p.161.
What is the independent variable in a study examining the effects of ultrasound on pain in patients with tennis elbow?
Ultrasound.
What is the dependent variable in the study examining the effects of ultrasound on pain in patients with tennis elbow?
Pain measured using the Visual Analog Scale.
What are some factors that can influence internal validity?
- Measurement errors/bias
- Subject selection
- History
- Regression to the mean
- Maturation
- Attrition
What is history in internal validity?
Outside events that occur after the initial assessment that may influence subsequent measures.
How can we minimize the effects of history on internal validity?
- Control group
- Random allocation of subjects to group
What is maturation in internal validity?
Passage of time: Changes in condition due to the normal time course of the condition or change in age.
How can we minimize the effects of maturation?
- Control group
- Random allocation of subjects to group
- Multiple testing at baseline
What is attrition in internal validity?
Subjects dropping out of a study.
Causes:
* Side effects experienced by treatment group
* Perceived lack of benefit in control group
Why is it important to understand the reasons behind attrition?
To assess whether the dropouts differ in characteristics from those who stayed, potentially affecting the study’s outcomes.
What is regression to the mean in internal validity?
A problem when using measurement tools with low reliability, where extreme scores at pretest tend to move closer to the mean on the second test.
What is subject selection in internal validity?
Important differences between groups exist prior to data collection, which can affect the interpretation of results.
How can we minimize selection bias?
- Random allocation of subjects to groups.
What are the three sources of measurement errors/bias?
- The instrument itself
- Tester
- Subject
How can we minimize instrument error/bias?
Use reliable and valid instruments.
What can lead to tester bias?
- Inexperience in using the instrument
- Differential elicitation, recording, or interpretation of information between groups.
How can we minimize tester bias?
- Well trained testers
- Standardized procedures
- Control group
- Blinding the tester
What is testing/practice effect in subject bias?
With repeated testing, subjects’ responses or measurements can change.
What is recall bias in subject bias?
Occurs when subjects who experienced a particular disorder remember their exposure history differently than those who are not affected.
What is desirability bias in subject bias?
Subjects want to please the testers, knowing that the tester would like to see a certain result.
What is the placebo effect in subject bias?
Changes due to being part of the study, where ‘treatment’ has no real physiological effect.
How can we minimize measurement bias from subjects?
- Practice trial before data collection
- Control group
- Blinding the subjects
- Placebo control group and random allocation
What is a major threat to internal validity in a study where many subjects participated in a relaxation program in addition to a counseling program?
Confounding variables.
What is a major threat to internal validity in a study where the exercise group was instructed differently than the control group during a walking test?
Measurement bias.
