Chapter 8

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12 Dec 2012
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Chapter 8 Experimental Design
Confounding and Internal Validity
- researcher manipulates independent and measures how dependent has changed
- confounding variable is a variable that varies along with the independent variable, a
third variable that can account for the dependent variable to change
- when the results of an experiment can confidently be attributed to the effect of an
independent variable, is said to have internal validity
- to achieve internal validity, the researcher must design and conduct the experiment so
that only the independent variable can be the cause of the results
Basic Experiments
- simplest experimental design has two variables, independent and dependent
- independent variable has two levels, experimental group and control group
- the researcher must make every effort to ensure that the only difference between the two
groups is the manipulated variable
- experimental method involves control over extraneous variables, through keeping such
variables constant or using randomization so that any extraneous variables affect both
groups randomly
- simple experimental design can take one of two forms, posttest-only design and pretest-
posttest design
Posttest-Only Design
- researcher does three things:
obtain two equivalent groups of participants
introduce the independent variable
measure the effect of the independent variable on the dependent variable
Pretest-Posttest Design
- the only difference between the posttest-only design and the pretest-posttest design is that
in the latter, a pretest is given before the experimental manipulation is introduced
- this design makes it possible to ascertain that the groups were in fact equivalent at the
beginning of the experiment
- rule of thumb is a minimum of 20-30 participants per condition (control and experimental
group)
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