Central Tendency and Variability Chapter 2 Descriptive Statistics Lecture 2 Jaymie Ticknor Quantitative Methods 2317 Sect 001 17 January 2014 Central Tendency values clump toward middle middle of a group of scores Variability spread out from middle Mean average of all scores most common M X N Mode most common value in a distribution largest frequency used with nominal variables Median middle score with numbers in order from lowest to highest used with rank ordered variable and skewed sometimes used when distributions have outliers score with an extreme value in relation to other scores Central Tendency use of mean median and mode in psychological research Normal Distribution mean median and mode in similar place center Positively Right Skewed left to right mode median and then mean tail Negatively Left Skewed left to right tail mean median and then mode Variability Variance The amount of spread of the scores around the mean how close or far from the mean are the scores more spread out higher variability 2 methods variance SD2 and standard deviation SD Steps subtract the mean M from each score square each of these deviation scores add up the squared deviations scores divide the sum of squared deviations SS X M 2 by the number N of scores SD2 X M 2 N SS N Standard Deviation Variance is calculated in squared deviations from the mean Most common way to describe a spread of scores is the standard deviation which is the square root of the variance
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