PSYC 1010 Lecture Notes - Confounding, Random Assignment, Scatter Plot

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15 May 2011
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Experimental Method (contd)
Study time: independent variable
Grade: dependent variable
Control group studies 0 hours; other groups study 5 and 10
Study time Grade
0 56
5 72
10 58
Hypothesis was not supported; more study time didnt equal higher grade
Another variable not the same – last group got no sleep the night before
If you fix that your study can be repeated
Confounding variable – a variable that interferes with the results of the study. It affects the dependent variable too
(alongside the independent variable), so you don’t know whether the effects are caused by the independent variable,
the confounding variable, or both.
In an experiment, you want to be able to say that the I.V. is the cause for the D.V.
A confounding variable interferes with this.
Ex: expectation can be a confounding variable in a study with a control group which is not a placebo control group
Extraneous variable – also applies to the experiment, but isnt an issue.
Ex: noise in the background of a reaction time study; it can become confounding when only one group is exposed to
the noise
There can be a problem if you put people into groups according to what they prefer. There may be a factor which
separates them from the rest of the population which will bias the results of the study.
Random assignment must be used to distribute subjects.
Random Assignment to Groups
Random selection is not the same as random assignment;
By using random assignment, you are assuming that allindividual difference” variables are being evenly distributed
among groups so that they are essentially equal.
Thus, not every person taking part in a study is equal; the groups average out to be (about) equal.
Individual difference variables can make groups unequal; these variables can potentially become confounding
variables. (You make the assumption that random assignment means equal groups)
If this doesnt work out (and you find out after the fact), you can a) control the variable in question or b) introduce
the confounding variable as an additional independent variable.
Correlational Method of Research
Experimental method – IV, DV, Control group, random assignment
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