July+9th+slides

# July+9th+slides - Quantitative Methods in Psychology July...

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Quantitative Methods in Psychology July 9th, 2009 Jeff Vietri Corresponding to chapter 3 in the text

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Today's Agenda Review homework Answer questions Quiz! Chapter 3: Measures of central tendency - Mean - Median - Mode Don't forget Monday's homework
Defining and Summarizing Distributions Central tendency/average Describes data with a single value There are a number of central tendencies - Mean - Median - Mode

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The mean Add up the scores and divide by the sample size Mu (population mean): X-bar (sample mean): X M n = X N μ=
The mean ∑(X – M) will always = 0! If we have M, n, and all but one datum, we can find the last datum

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The weighted mean How should one combine the means from two groups? - M1 = 35, n1 = 7 - M2 = 27, n2 = 3 Two methods - (∑X1 + ∑X2) / n-total (245+81)/10 - M1(n1/(total n)) + M2(n2/(total n)) 35(7/10) + 27 (3/10)
Constants Adding/subtracting constants - If we add the same number to every score, then

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July+9th+slides - Quantitative Methods in Psychology July...

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