Variability mini-lecture

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Published on January 22, 2018

Author: jennrip

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Variability: Variability Measures of variability and computing Variance and Standard Deviation Dr. Jennifer Ripley, Regent University Measures of Variability: Measures of Variability Three Measures of Variability Range Standard Deviation Variance Typically report the average and the variability together to describe a distribution. Salkind, Statistics for People Who (Think They) Hate Statistics 5th Edition, SAGE Inc. © 2014 Computing Standard Deviation: Computing Standard Deviation Most frequently used measure of variability SD = s= average amount of variability in a set of scores What do these symbols represent?   Salkind, Statistics for People Who (Think They) Hate Statistics 5th Edition, SAGE Inc. © 2014 Why n – 1?: Why n – 1? Standard deviation is an estimate of the POPULATION standard deviation To make it an “unbiased estimate” for the SAMPLE you must subtract 1 from n This artificially inflates the SD (it makes it bigger) because it makes the denominator smaller Salkind, Statistics for People Who (Think They) Hate Statistics 5th Edition, SAGE Inc. © 2014 Things to Remember…: Things to Remember… Standard deviation is computed as the average distance from the mean for your sample The larger the standard deviation the more spread out the values are in your variable Like the mean, the standard deviation is sensitive to extreme scores or outliers If s = 0, then there is no variability among scores and the scores are identical in value Salkind, Statistics for People Who (Think They) Hate Statistics 5th Edition, SAGE Inc. © 2014 Computing Variance: Computing Variance Variance = how much a variable varies standard deviation squared So…what do these symbols represent? Does the formula look familiar? Salkind, Statistics for People Who (Think They) Hate Statistics 5th Edition, SAGE Inc. © 2014 Standard Deviation or Variance: Standard Deviation or Variance While the formulas are quite similar…the two are also quite different. Standard deviation is stated in original units Variance is stated in units that are squared Which do you think is easier to interpret??? Salkind , Statistics for People Who (Think They) Hate Statistics 5th Edition, SAGE Inc. © 2014 Using SPSS to Compute Measures of Variability: Using SPSS to Compute Measures of Variability Salkind, Statistics for People Who (Think They) Hate Statistics 5th Edition, SAGE Inc. © 2014

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