Summary Kazdin – Research Design in Clinical Psychology
2. Internal and external validity
- The important concept of plausible rival hypothesis addresses those competing
interpretations that might be posed to explain the findings of a particular study.
Methodology helps rule out or at least make implausible competing interpretations
2.1 Types of validity
- The purpose of research is to reach well-founded (i.e. valid) conclusions about the effects
of a given experimental manipulation or intervention. Four categories of types of
experimental validity organize the many concepts used to evaluate the methodology of a
study. These include:
o Internal validity to what extent can the intervention rather than extraneous
influences be considered to account for the results, changes, or differences among
conditions?
o External validity to what extent can the results be generalized or extended to
people, settings, times, measures/outcomes, and characteristics other than those
included in this particular demonstration?
o Construct validity given that the experimental manipulation or intervention
was responsible for change, what specific aspect of the manipulation was the
mechanism, process, or causal agent? What is the conceptual basis (construct)
underlying the effect?
o Data-evaluation validity to what extent is a relation shown, demonstrated, or
evident between the experimental manipulation or intervention and the outcome?
What about the data and methods used for evaluation that could mislead or
obscure demonstrating or failing to demonstrate and experimental effect?
- A decision to maximize one type of validity can be at the expense of others
2.2 Internal validity
- Independent variables are the experimental manipulation or variable we are evaluating
to see if it has an effect
- Dependent variables are the outcome or measure we are examining to reflect the impact
or effects of the independent variable
- When the results can be attributed with little or no ambiguity to the effects of the
independent variable, the experiment is said to be internally valid
o Internal validity refers to the extent to which an investigation rules out or make
implausible alternative explanations of the results
2.3 Threats to internal validity
- History
o History refers to any event, other than the independent variable, occurring in the
experiment or outside of the experiment that may account for the results. Historical
events might include family crises; change in job, teacher, or spouse; power blackouts;
or any other event
o This is a systematic bias or experience that the subjects receive while in the
experiment that could explain how they responded on the dependent measures
- Maturation
o Maturation refers to processes within the participants that change over time and
includes growing older, stronger, wiser, and more tired or bored.
ECP - Page 1 of 39
2. Internal and external validity
- The important concept of plausible rival hypothesis addresses those competing
interpretations that might be posed to explain the findings of a particular study.
Methodology helps rule out or at least make implausible competing interpretations
2.1 Types of validity
- The purpose of research is to reach well-founded (i.e. valid) conclusions about the effects
of a given experimental manipulation or intervention. Four categories of types of
experimental validity organize the many concepts used to evaluate the methodology of a
study. These include:
o Internal validity to what extent can the intervention rather than extraneous
influences be considered to account for the results, changes, or differences among
conditions?
o External validity to what extent can the results be generalized or extended to
people, settings, times, measures/outcomes, and characteristics other than those
included in this particular demonstration?
o Construct validity given that the experimental manipulation or intervention
was responsible for change, what specific aspect of the manipulation was the
mechanism, process, or causal agent? What is the conceptual basis (construct)
underlying the effect?
o Data-evaluation validity to what extent is a relation shown, demonstrated, or
evident between the experimental manipulation or intervention and the outcome?
What about the data and methods used for evaluation that could mislead or
obscure demonstrating or failing to demonstrate and experimental effect?
- A decision to maximize one type of validity can be at the expense of others
2.2 Internal validity
- Independent variables are the experimental manipulation or variable we are evaluating
to see if it has an effect
- Dependent variables are the outcome or measure we are examining to reflect the impact
or effects of the independent variable
- When the results can be attributed with little or no ambiguity to the effects of the
independent variable, the experiment is said to be internally valid
o Internal validity refers to the extent to which an investigation rules out or make
implausible alternative explanations of the results
2.3 Threats to internal validity
- History
o History refers to any event, other than the independent variable, occurring in the
experiment or outside of the experiment that may account for the results. Historical
events might include family crises; change in job, teacher, or spouse; power blackouts;
or any other event
o This is a systematic bias or experience that the subjects receive while in the
experiment that could explain how they responded on the dependent measures
- Maturation
o Maturation refers to processes within the participants that change over time and
includes growing older, stronger, wiser, and more tired or bored.
ECP - Page 1 of 39
