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Chapter 13

PS295 Chapter 13

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Roger Buehler

Chapter 13: Quasi-Experimental Research  Quasi-experimental designs: are used are used if the researcher lacks control over the assignment of participants to conditions and or does not manipulate the causal variable of interest  Quasi-experimental variable: indicates the variable that is not a true independent variable that is manipulated by the researcher but rather is an event that participants experienced for other reasons  This type of research has a decreased level of internal validity because participants are not randomly assigned to the circumstances The quasi-experimental designs go in order from least preferable/valid to most * Pretest-Posttest designs:  Ex. 01 x 02 o 01 is pretest, x is program or variable and 02 is posttest meaure  One group pretest-posttest design is a very poor research study – fails to eliminate most threats to internal validity o Is a preexperimental design: lacks control, no internal valditiy- fails to meet a research design  This design can be threatened by regression to the mean: the tendency for extreme scores in a set of data to move, or regress, toward the mean of the distribution with repeated testing Non-equivalent control group design: the researcher looks for one or more groups of participants that appear to be reasonably similar to the group that received quasi-independent variable  Non-equivalent groups posttest-only design: o Issue is that there is no way of telling if the groups were similar before the group received the treatment o Quasi-experimental group X O , non-equivalent control group – O  Non-equivalent groups pre-test – post-test design: o Quasi-experimental group 01 x 02, non-equivalent 01 – 02 o This design allows us to see whether the 2 groups scored similarly on the dependent variable before the intro of the treatment at point x Time Series Designs:  Weaknesses in the non-equivalent control groups can be eliminated by this design  Time series designs measure the dependent variable on several different occasions before and on several occasions after the quasi-independent variable occur o Further threats to internal validity can be eliminated Simple interrupted time series design:  Take several pretest measures before introducing independent (or quasi-independent) variable and then taking several posttest measures afterward o Shown like: 01 02 03 04 X 0
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