April 14 - interested in(will use two-tail most often but...

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April 14, 2008 Cont. from April 11 Intro to Inferential statistics: April 11-16 The t-distribution o The Z distribution used above assumes a large sample (N>100) o If the sample is smaller, we will have a larger standard error and less precision in out estimate o For smaller samples, we use a t-distribution o Has same overall properties, however its flatter o Degree of freedom is n-1 o The t-distribution looks like the z-distribution o But: It has more variability – it is flatter and has wider tails It changes based on the number of cases (degrees of freedom =n-1) Appendix E-3, p 725 To read, need to know: o What level Alpha we are using (far left side); usually α=.05 o Degrees of freedom (n-1) o One tail test or two tail test; which side of the curve are we
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Unformatted text preview: interested in? (will use two-tail most often but not always) • E.g. 16 cases o Degree of freedom is 15 o Alpha of .05 o Two tail test o Value is 2.131 Calculating the confidence intervals with the t-distribution o CI = ± t x (s/ n ) o Now we need to know both our alpha (usually .05) and our degrees of freedom (DF=n-1) o Always use two-tail test with confidence interval o Example (student GPAs) n=30 = 2.7 S x = .8 DF = n-1 = 30-1 = 29 t critical = ± 2.045 (from Appendix E-3) 95% = ± t (S x / n ) = 2.7 ± 2.045 (.8/ 30 ) = 2.7 ± 2.045 (.146) = 2.7 ± .30 = 2.4 to 3.0...
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