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Political Science 3N06 2013 L10.docx

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Political Science
Todd Alway

Experimental methods Political Science 3N06 2013 Lecture 10: Pre-test-------------independent variable------------Post test  Measure your dependent variable- apply independent variable  problems of inference- there can be some internal validity issues  the environment can be affected by more external factors that may effect the dependent variable  external variables might affect it; maturational changes over the course of the experiment  they can make our internal validity rather low  raises certain questions; if you want to be more confident to how to rule out all the other variables? introduce a control group o separate set of subjects o the control group will not be affected by the independent variable o they have to be exactly the same as the experimental group o need to equalize the two groups o the independent variable accounts for it o the control group helps to knock out other variables o if the groups differ; than external variables will have different affects  Some of the limitations of a one group pretest/post-test design can be countered by using a control group o A group that should resemble the experimental group in every aspect but one – it is not exposed to the independent variable  Scores between the control group and the experimental group can be compared – with the difference being attributed to the independent variable  Using a control group can increase the internal validity of an experiment  Threats to the internal validity of an experiment continued:  9. Selection Bias o Control groups only form a control when they are effectively equivalent to the experimental group o “Selection bias” (in this context) implies that the control and experimental group are not equivalent o If so, then the difference in scores in the post-test (between groups) may be reflecting the difference in subjects, not the effect of the independent variable that we are interested in o the internal validity is at threat if the control group and experimental group are different  Ways to enhance pre-test group equivalence A) Probability sampling o We will discuss probability sampling next lecture o random sampling; take random samples and split them within the two groups o it is difficult to do; hard to get people o there are certain limitations B) Random assignment o Individuals in the sample are assigned to the control or the experimental group on the basis of random chance alone o As long as the groups are of a sufficient size, random assignment should equalize their average characteristics o random group; and divide them randomly in the control and experiment group o any purely random process; using chance and chance alone o it is more likely to equalize the groups o the size of the group has to be significantly large o have at least 30 subjects o works better; when the group is larger o the bigger your sample the less likely the mismatch of data C) Matching o Where groups are small, random assignment may not work to equalize the experimental and control groups o In that case, matching individuals into pairs (on the basis of shared characteristics)  And then splitting the pairs – with one member going into the control and one into the experimental group  May be the more appropriate option o Although even here – which characteristics should be relevant for the matching process? o when you cannot rely on random chance to make the two groups equal o tempt to match the experimental group in pares to separate
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