Inferential StatisticsInferential Statistics

Significance TestingSignificance Testing

Chapter 4Chapter 4

Important TermsImportant Terms

• Null hypothesis

• Significance

• Probability of error (alpha levels)

• Confidence level

• Independent t test

• Dependent t test

AssumptionsAssumptions

• Group studies – comparing experimental group with control group

• Representative values to describe performance in both groups – mean and Sd

• Probability of replication

Comparing Mean ValuesComparing Mean Values

• Mean values will be different

• Caused by chance or caused by IV

Null HypothesisNull 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

CalculationCalculation

• Independent t-test

• Non independent t-test

• See pp. 87-109 in supplemental readings

Independent t-testIndependent 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?

SignificanceSignificance

• 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 LevelsAlpha 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. 307 in textbook Appendix C

Alpha levels (cont.)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-testNon independent t-test

• To determine significance between pre and post tests within one group

• One group, two treatments

• Examples