Training Course on Basic Statistics for Research August 24-28, 2009 STATISTICAL RESEARCH AND TRAINING CENTER J and S Building, 104 Kalayaan Avenue, Diliman,

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Training Course on Basic Statistics for Research August 24-28, 2009 STATISTICAL RESEARCH AND TRAINING CENTER J and S Building, 104 Kalayaan Avenue, Diliman, Quezon City Measures of Skewness Prepared by: Josefina V. Almeda Professor and College Secretary School of Statistics University of the Philippines, Diliman August 2009

2 Statistical Research and Training Center Training Course on Basic Statistics for Research August , 2009 Symmetric Distribution * a distribution is symmetric if it can be folded along the vertical axis so that the two sides coincide * if the distribution is symmetric, the mean, the median, and the mode are equal and are located at the same position along the horizontal axis

3 Statistical Research and Training Center Training Course on Basic Statistics for Research August , 2009

4 Statistical Research and Training Center Training Course on Basic Statistics for Research August , 2009 Skewed Distribution * if the two sides do not coincide, distribution is said to be asymmetric * a distribution that is asymmetric with respect to a vertical axis is said to be skewed.

5 Statistical Research and Training Center Training Course on Basic Statistics for Research August , 2009 Two Types of Skewness Positively Skewed or Skewed to the Right Distribution * distribution tapers more to the right than to the left * has a longer tail to the right compared to a much shorter left tail * values are more concentrated below than above the mean * large values in the right tail are not offset by correspondingly low values in the left tail and consequently the mean will be greater than the median

6 Statistical Research and Training Center Training Course on Basic Statistics for Research August , 2009

7 Statistical Research and Training Center Training Course on Basic Statistics for Research August , 2009 Negatively Skewed or Skewed to the Left Distribution * distribution tapers more to the left than to the right * has longer tail to the left compared to a much shorter right tail * values are more concentrated above than below the mean * small values in the left tail will make the mean less than the median

8 Statistical Research and Training Center Training Course on Basic Statistics for Research August , 2009

9 Statistical Research and Training Center Training Course on Basic Statistics for Research August , 2009 Measure of skewness * shows the degree of asymmetry, or departure from symmetry of a distribution * indicates also the direction of the distribution

10 Statistical Research and Training Center Training Course on Basic Statistics for Research August , 2009 Pearson’s First and Second Coefficient of Skewness where= mean, Mo = mode, s = standard deviation SK = Remarks: 1.Since the mode is frequently only an approximation, formula 2 is preferred. 2.Interpretation of the measure of skewness: where Md = median, s = standard deviation SK > 0:positively skewed since> Md > Mo SK < 0: negatively skewed since< Md < Mo

11 Statistical Research and Training Center Training Course on Basic Statistics for Research August , 2009 Example: Given the following statistics on weights of court justices in kilograms: =74.1Md = 75Mo = 84s = Using the first formula, SK = (skewed to the left) Using the second formula, SK =(skewed to the left)

Training Course on Basic Statistics for Research August 24-28, 2009 STATISTICAL RESEARCH AND TRAINING CENTER J and S Building, 104 Kalayaan Avenue, Diliman, Quezon City Thank you.