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Lecture 4 Psyc 300A Measures of Central Tendency • Mode: The most frequently occurring score • Median: The midpoint in an ordered set of scores. The 50th percentile. • Mean: Sum of all scores divided by the number of scores. • Which measure to use? Measures of Dispersion • Range: Highest score minus lowest score. • Standard Deviation: Average amount of variability in a set of scores. Technically, the square root of variance. • Variance: Average squared deviation from the mean. Formulas to Calculate the SD Standard Deviation X _ (X – X) _ (X – X)2 8 8 8 7 6 6 5 5 4 3 +2 +2 +2 +1 0 0 -1 -1 -2 -3 4 4 4 1 0 0 1 1 4 9 sum 0 28 s = Sqrt (28/n-1)= 1.76 Using SPSS • What is SPSS? • Setting up a data file • Running a frequency analysis with descriptive statistics