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Independent Sample T-test Classical design used in psychology/medicine N subjects are randomly assigned to two groups (Control * Treatment). After treatment, the individuals are measured on the dependent variable. A test of differences in means between groups provides evidence for the treatment's effect.

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Measures of Variation A lot of statistical techniques (using interval data) use measures of variation in some manner What is the difference between a standard deviation, the standard error of the mean, and the standard error of the difference between means? Or How are they related? Look in the glossary to help you answer these questions?

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Using Measures of Variation Leaned how to measure variation in data, i.e., variance, standard deviation (Ch.4) Used the normal curve & SD to calculate z-scores and probabilities (Ch.5) Used the normal curve & the z-score & the SE of the mean to calculate confidence intervals (Ch.6) Used the concept of the confidence interval and the standard error of the differences between means to calculate the t-test (Ch.7) Use the sum of squares Σ(X – Mean) 2 [sum of the squared differences from the mean] in ANOVA

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Null Hypothesis The two groups come from the same population or that the two means are equal μ 1 = μ 2

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Levels of Significance What does an α =.05 level of significance mean? We decide to reject the null if the probability is very small (5% or less) that the sample difference is a product of sampling error. The observed difference is outside the 95% confidence interval of the difference

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Choosing a Level of Significance Convention Minimize type I error – Reject null hypothesis when the null is true Minimize type II error – fail to reject null when the null is false Making alpha smaller reduces the likelihood of making a type I error Making alpha larger reduces the probability of a type II error

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Independent Sample T-test Formula t =

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Assumptions of the t-test 1. All observations must be independent of each other (random sample should do this) 2. The dependent variable must be measured on an interval or ratio scale 3. The dependent variable must be normally distributed in the population (for each group being compared). (NORMALITY ASSUMPTION) [this usually occurs when N is large and randomly selected] 4. The distribution of the dependent variable for one of the groups being compared must have the same variance as the distribution for the other group being compared. (HOMOGENEITY OF VARIANCE ASSUMPTION)

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Don’t worry about these assumptions to much, but Point 1: statistical tools are attempting to quantify and analyze very complex social/political phenomenon Point 2: For these test to be accurate they relay on simplifying the world with many assumptions that might not be true Point 3: social science researchers violate these assumptions quite often, but try to be honest about it Point 4: there are sometimes ways of testing and adjusting for violations

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SPSS & the Independent Sample T-Test

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