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t -Tests: Measuring the Differences Between Group Means

t -Tests: Measuring the Differences Between Group Means. t-test. Comparing group means What is the probability of getting a difference this large by chance alone?. t-test. Type of Data Required - Nominal-level independent variable with two levels

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t -Tests: Measuring the Differences Between Group Means

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  1. t-Tests: Measuring the Differences Between Group Means

  2. t-test • Comparing group means • What is the probability of getting a difference this large by chance alone?

  3. t-test • Type of Data Required - Nominal-level independent variable with two levels - Continuous dependent variable

  4. t-test • Assumptions - Two mutually exclusive groups - Normally distributed dependent variable - Homogeneity of variance

  5. t-tests • Formulas • Pooled or equal variance • Separate or unequal variance • Correlated or paired

  6. t-test • Alpha level • Power • One vs. two-tailed • Effect size • Sample size

  7. Cohen’s Effect Sizes - t-test • Small = .2 • Moderate = .5 • Large = .8

  8. The One-Sample T Test procedure tests whether the mean of a single variable differs from a specified constant. • Examples. A researcher might want to test whether the average IQ score for a group of students differs from 100. Or, a cereal manufacturer can take a sample of boxes from the production line and check whether the mean weight of the samples differs from 1.3 pounds at the 95% confidence level.

  9. The Independent-Samples T Test procedure compares means for two groups of cases. • Ideally, for this test, the subjects should be randomly assigned to two groups, so that any difference in response is due to the treatment (or lack of treatment) and not to other factors. • Differences in average income may be influenced by factors such as education and not by gender alone.

  10. Independent groups T Test • Example. Patients with high blood pressure are randomly assigned to a placebo group and a treatment group. The placebo subjects receive an inactive pill and the treatment subjects receive a new drug that is expected to lower blood pressure. • After treating the subjects for two months, the two-sample t test is used to compare the average blood pressures for the placebo group and the treatment group. Each patient is measured once and belongs to one group.

  11. Effect sizes • GRE Scores • Mean = 500 • Standard Deviation = 100

  12. the students’ perceptions and differences by gender showed that female students had a significantly higher overall mean score of nursing perceptions, t = -4.05, d.f. = 603 (P 0.00)

  13. Effect sizes • LOPSS • Difference between groups = 17 points • Standard deviation = 24

  14. Exercises • 1. Do individuals who have drug A differ from those who have Placebo on the following two measures: Heart Rate and Weight? • 2. Do current ratings of satisfaction with weight differ significantly from ratings of weight at age 18?

  15. SPSS - t-test, 2 groups • ANALYZE • Compare Means • Independent Samples T Test

  16. SPSS - t-test, one group measured twice • ANALYZE • Compare Means • Paired Samples T Test

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