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Inferential Statistics. Significance Testing Chapter 13. Important Terms. Null hypothesis Significance Probability of error (alpha levels) Confidence level Independent t test Dependent t test. Assumptions. Group studies – comparing experimental group with control group
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Inferential Statistics Significance Testing Chapter 13
Important Terms • Null hypothesis • Significance • Probability of error (alpha levels) • Confidence level • Independent t test • Dependent t test
Assumptions • Group studies – comparing experimental group with control group • Representative values to describe performance in both groups – mean and Sd • Probability of replication
Comparing Mean Values • Mean values will be different • Caused by chance or caused by IV
Null Hypothesis • Any observable difference between two mean values is simply due to chance • Significance testing either accepts or rejects null hypothesis • Acceptance means that observable difference was due to chance • Rejection means the IV caused the change
Calculation • Independent t-test • Non independent t-test • See p.335
Independent t-test • To determine significance between two independent groups, i.e., experimental group and control group • Experimental group receives IV; control group does not • Was the difference between experimental group mean and control group great enough to state significance?
Significance • In essence, what is the likelihood that the group that receives the IV will score higher than the group who does not? • In essence, what is the probability that the group who receives the IV will not score higher than the group who does not? • In essence, how confident are you that the group who receives the IV will score higher than the group who does not?
Alpha Levels • The symbol p means probability of error • An alpha level of .05 means probability of error is less than 5 out of 100 times that the IV group might not score higher • Or, 95% of the times the IV group will score higher • See p. 329
Alpha levels (cont.) • An alpha level of .01 means that less than 1 out of 100 times the IV group might not score higher • Or, 99% of the times the IV group will score higher • What about an alpha level of .001?
Non independent t-test • To determine significance between pre and post tests within one group • One group, two treatments • Examples