Internal and External Validity [PDF]

When research is designed to investigate cause and effect relationships (explanatory research) through the direct manipu

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Idea Transcript


Internal and External Validity ScWk 240 Week 5 Slides (2nd Set)

1

Defining Characteristics —  When research is designed to investigate cause and

effect relationships (explanatory research) through the direct manipulation of an independent variable and control of extraneous variables. Review of Terms:

—  Independent variable – the variable being

manipulated —  Dependent variable – the variable in which the effect of the manipulation of the independent variable is observed —  Researcher manipulation and control – choice of treatments, choice of a research design, use of specific procedures, etc. 2

Selecting Your Research Question(s) Consider using the FINER Framework Is Your Research Question:

— Feasible — Interesting — Novel — Ethical — Relevant 3

Manipulation in Research Manipulation —  The researcher’s decisions related to what

constitutes the independent variable —  Active and assigned variables —  Active variables are those the researcher actively manipulates —  Choice of an instructional strategy —  A particular intervention approach —  Assigned variables are those that cannot be manipulated by the researcher but are of interest: —  Gender —  Race 4

Control in Research Control —  The researcher’s efforts to remove the influence of any

extraneous variables that might have an effect on the dependent variable —  The goal is to be assured the only differences between groups is that related to the independent variable —  Participant variables – characteristics of the subjects —  Pre-existing functioning levels —  Differences in attitudes —  Environmental variables – characteristics of the context —  Intervention materials —  Differences in the time available for treatment between groups 5

Reliability —  Implies that the same data would have

been collected each time over repeated tests/ observations.

—  Would a particular technique (or survey

question) yield the same result each time? —  “Did you go to your support group last week?” vs. “How many times have you been to these support groups in your life?”

—  Reliability does not ensure accuracy. —  Taken from Babbie, E. 6

External and Internal Validity —  Internal Validity – the degree to which the results are attributable to the independent variable and not some other rival explanation

—  External Validity – the extent to which the results of a study can be generalized 7

Threats to Internal and External Validity – Questions: ■ Are the investigator’s conclusions correct? ■ Are the changes in the independent variable indeed responsible for the observed variation in the dependent variable? ■ Might the variation in the dependent variable be attributable to other causes? 8

Causal Inference Three conditions of causality: 1.  Cause precedes the effect 2.  Cause and effect must correlate 3.  No third variable involved 9

Correlations Relationships between variables can be either: —  Strong or weak —  Positive or negative Strongest (perfect) positive correlation is +1 Strongest (perfect) negative correlation is -1 No correlation (unrelated variables) is 0 A weak positive relationship is 0.2 A weak negative relationship is -0.2 10

Internal Validity Internal Validity §  Confidence that changes in Dependent (DV) Variable are actually caused by the Independent Variable (IV)

v Validity (in measurement) 11

Why is Internal Validity Important? ■ We often conduct research in order to determine cause-and-effect relationships. ■ Can we conclude that changes in the independent variable caused the observed changes in the dependent variable? ■ Is the evidence for such a conclusion good or poor? ■ If a study shows a high degree of internal validity then we can conclude we have strong evidence of causality. ■ If a study has low internal validity, then we must conclude we have little or no evidence of causality 12

Internal Validity (Cont.) Eight Threats to Internal Validity: §  Factors other than IV affects DV: 1.  History 2.  Maturation (passage of time) 3.  Testing 4.  Instrumentation 13

Internal Validity (Cont.) Eight Threats to Internal Validity (Cont.): 5.  Statistical regression 6.  Research reactivity 7.  Selection biases 8.  Attrition (experimental mortality) 14

External Validity §  Generalizability §  Representativeness of sample, setting and procedures §  Sampling and survey research 15

Threats to External Validity —  Pre-test treatment interaction —  Multiple treatment interference —  Selection treatment interaction —  Specificity of variables —  Participants —  Operational definition of the treatment —  Operational definition of the dependent variable —  Specific times —  Specific circumstances

—  Treatment diffusion and inconsistencies 16

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