lesson 1 additional

• Measure of central tendency


  A descriptive statistic that describes the average, or typical value of a set of scores
• Common measures of central tendency

  • The mode
  • The median
  • The mean
• Mode


  The score that occurs most frequently in a set of data
• Bimodal distribution


  • A distribution has two "modes"
• Multimodal distribution


  • A distribution has more than 2 "modes"
• When to use the mode


  The mode is not a very useful measure of central tendency as it is insensitive to large changes in the data set
• When to use the mode

  The mode is primarily used with nominally scaled data as it is the only measure of central tendency that is appropriate for nominally scaled data
• Median


  The 50th percentile, the score in the middle where half of the scores are larger and half are smaller
• How to calculate the median

  1. Sort the data from highest to lowest

  2. Find the score in the middle

  3. If N is even, the median is the average of the middle two scores
• When to use the median

  The median is often used when the distribution of scores is either positively or negatively skewed as the few really large or small scores will not overly influence the median
• Mean


  The arithmetic average of all the scores, the number that makes the sum of the deviations from it equal to 0, and the number that makes the sum of the squared deviations a minimum
• Calculating the mean


  1. Sum the scores (X)

  2. Divide the sum (X) by the number of scores (N)
• When to use the mean


  You should use the mean when the data are interval or ratio scaled and the data are not skewed, as the mean is sensitive to every score
• In symmetrical distributions


  The median and mean are equal
• For normal distributions


  Mean = median = mode
• In positively skewed distributions


  The mean is greater than the median
• In negatively skewed distributions

  The mean is smaller than the median