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Published byDominic Carr Modified over 8 years ago
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Statistical Significance
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Office Hour Sign Up I’d like to meet with everybody 1 on 1 re papers Please sign up during office hours, or let me know If those times don’t work
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The Normal Distribution Hypothetical Distribution Mean=Median=Mode Symmetrical Fixed proportion of cases between mean and a distance from the mean
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The Normal Curve
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An example Suppose the average income were $50K, SD=15,000 Where does a person with income of $80K fall? (80,000-50,000)/15,000= 2 SD above mean
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The Normal Curve Previous case- 2 SDs, where does that put us? 50% of curve from - ∞ to mean ~48% of curve from mean to 2 SD Higher than 98 Percent of cases
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Sampling Error Mean from a sample is an ESTIMATE of the population parameter Standard Error- estimate of precision of mean
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Sampling Error- Example George W Bush Feeling thermometer mean= 56.1, SD=24.9, N=1761 How precise of an estimate is this of population mean? SE=24.9/√1761=.59 95% Confidence Interval- Add and subtract 2SE ~54-57.3
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Hypothesis Testing State Null Hypothesis Choose decision rule (level of significance) Choose appropriate sample test statistic Collect data, put it into SPSS, go get a cup of coffee See if test statistic is meets level of significance
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How to tell if groups differ part I Chi Square Question- Suppose we have two groups, how do we tell if they are different on a variable of interest? Χ 2 – Another distribution Works with nominal data Where to use- Crosstabs Compares expected number of cases if no difference between groups with observed number of cases
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Chi Square Example
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Pres Approval by Party
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Do Two Groups Differ- Interval Data Difference of Means T test Do two groups differ? Republicans and Democrats on feeling thermometers
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T Tests Can Also Compare Pairs of Variables E.g. do Clinton, Gore, Bush differ?
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Anova Limit of T test- can only compare 2 groups/variables Need one continuous, one dichotomous variable Anova works with two continuous variables to see if there is a relationship
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Feeling Thermometers and Partisanship NES 7 point Party Scale Captures not only difference between Republicans and Democrats, but also importance of strength No utility in looking at mean for each score on IV
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Anova
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Comparing Groups, which test when Nominal Data- Chi Square Compare 2 variables-Paired Samples T Test Dichotomous IV, Continuous DV- independent samples T-test 2 continuous Variables- Anova
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Statistical Significance and Measures of Association 0= no association Correlations etc have SEs as well Null hypothesis- score equals zero Significance tells us if it differs meaningfully from 0 Sample size and strength of association influence significance Strong Associations Big Samples
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Next week Learning SPSS A- McGrath Meet in SSC 27 Tuesday Mickelson-Z on Thursday Day off otherwise, do something productive
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