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Chapter S4 Measures of dispersion Learning Objectives • Calculate common measures of dispersion from grouped and ungrouped data (including the range, interquartile range, mean deviation, and standard deviation) • Calculate and interpret the coefficient of variation © 2002 McGraw-Hill Australia, PPTs t/a Introductory Mathematics & Statistics for Business 4e by John S. Croucher Slide 1 1 The range Simply the difference between the largest and smallest values in a set of data Useful for: daily temperature fluctuations or share price movement Is considered primitive as it considers only the extreme values which may not be useful indicators of the bulk of the population. The formula is: Range = largest observation - smallest observation © 2002 McGraw-Hill Australia, PPTs t/a Introductory Mathematics & Statistics for Business 4e by John S. Croucher Slide 2 2 Interquartile range Measures the range of the middle 50% of the values only Is defined as the difference between the upper and lower quartiles Interquartile range = upper quartile - lower quartile = Q3 - Q1 © 2002 McGraw-Hill Australia, PPTs t/a Introductory Mathematics & Statistics for Business 4e by John S. Croucher Slide 3 3 The mean deviation Measures the ‘average’ distance of each observation away from the mean of the data Gives an equal weight to each observation Generally more sensitive than the range or interquartile range, since a change in any value will affect it © 2002 McGraw-Hill Australia, PPTs t/a Introductory Mathematics & Statistics for Business 4e by John S. Croucher Slide 4 4 Actual and absolute deviations from mean A set of x values has a mean of The residual of a particular x-value is: Residual or deviation = x x - x The absolute deviation is: x-x © 2002 McGraw-Hill Australia, PPTs t/a Introductory Mathematics & Statistics for Business 4e by John S. Croucher Slide 5 5 Mean deviation The mean of the absolute deviations Mean deviation xx n © 2002 McGraw-Hill Australia, PPTs t/a Introductory Mathematics & Statistics for Business 4e by John S. Croucher Slide 6 6 To calculate mean deviation 1. Calculate mean of data Find x 2. Subtract mean from each For each x, find observation xx Record the differences 3. Record absolute value of Find each residual xx for each x 4. Calculate the mean of the absolute values Mean deviation xx n Add up absolute values and divide by n © 2002 McGraw-Hill Australia, PPTs t/a Introductory Mathematics & Statistics for Business 4e by John S. Croucher Slide 7 7 The standard deviation Measures the variation of observations from the mean The most common measure of dispersion Takes into account every observation Measures the ‘average deviation’ of observations from mean Works with squares of residuals not absolute values—easier to use in further calculations © 2002 McGraw-Hill Australia, PPTs t/a Introductory Mathematics & Statistics for Business 4e by John S. Croucher Slide 8 8 Standard deviation of a population δ Every observation in the population is used. 2 Standard deviation δ x x n The square of the population standard deviation is called the variance. Variance δ 2 © 2002 McGraw-Hill Australia, PPTs t/a Introductory Mathematics & Statistics for Business 4e by John S. Croucher Slide 9 9 Standard deviation of a sample s In practice, most populations are very large and it is more common to calculate the sample standard deviation. x x 2 Sample standard deviation s n 1 Where: (n-1) is the number of observations in the sample © 2002 McGraw-Hill Australia, PPTs t/a Introductory Mathematics & Statistics for Business 4e by John S. Croucher Slide 10 10 To calculate standard deviation 1. Calculate the mean 2. Calculate the residual for each x 3. Square the residuals x xx ( x x )2 2 2 2 4. Calculate the sum of the squares xx 5. Divide the sum in Step 4 by (n-1) xx n 1 6. Take the square root of quantity in Step 5 xx n 1 © 2002 McGraw-Hill Australia, PPTs t/a Introductory Mathematics & Statistics for Business 4e by John S. Croucher Slide 11 11 Standard deviations for frequency distributions If data is in a frequency distribution No. Units n 1 Frequency f 85 2 192 3 123 Total 400 Total Calculate standard deviation using: s x x 1 2 © 2002 McGraw-Hill Australia, PPTs t/a Introductory Mathematics & Statistics for Business 4e by John S. Croucher Slide 12 12 Coefficient of variation Is a measure of relative variability used to: – measure changes that have occurred in a population over time – compare variability of two populations that are expressed in different units of measurement – expressed as a percentage rather than in terms of the units of the particular data © 2002 McGraw-Hill Australia, PPTs t/a Introductory Mathematics & Statistics for Business 4e by John S. Croucher Slide 13 13 Formula for coefficient of variation Denoted by V s V 100 % x where x = the mean of the sample s = the standard deviation of the sample © 2002 McGraw-Hill Australia, PPTs t/a Introductory Mathematics & Statistics for Business 4e by John S. Croucher Slide 14 14 Summary Measures of central tendency – no ideal measure of dispersion exists – standard deviation is the most important measure of central tendency • it is the most frequently used • the value is affected by the value of every observation in the data • extreme values in the population may distort the data © 2002 McGraw-Hill Australia, PPTs t/a Introductory Mathematics & Statistics for Business 4e by John S. Croucher Slide 15 15