PSYC 202 Chapter 9: PSYC202 Chapter 9 Into to t Statistic.docx
29 Jan 2014
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Because the sample mean places a restriction on the value of one score in the sample, there are n 1 degrees of freedom for the sample: the greater the value of df, the better s2 represents 2, and the better t approximates the z score. Larger sample = more accurate. t-distribution (cid:224) distribution of t-scores similar to z-scores both normal. How normal the distribution is depends on the degrees of freedom. Measuring the percentage of variance explained, r: alternative method for measuring effect size, to measure treatment effect size, calculate deviations from the mean and sum of squared deviations (ss, measuring how much variability reduces when treatment effect removed: r2 = percentage of total variability. Thus, removing the treatment effect reduces the variability by^ r2= t 2 t2+df: r2 = 0. 01 (cid:224) small effect, r2 = 0. 09 (cid:224) medium effect, r2 = 025 (cid:224) large effect, r2 only slightly influenced by sample size.
