I.C. Review: Describing Data Numerically

The two more often used set of measures to numerically describe data sets are the measures of

• central tendency
• Read David Lane's Introduction to Central Tendency and Measures of Central Tendency
• variability or spread
  • Read the four most often used measures of variability
  • Read about Mean Absolute Deviation, a rarely used measure of variability but one that makes most sense conceptually

Visit Jay Hill's Introduction to Descriptive Statistics to read more about the above-mentioned measures.

View an interactive illustration of the relationship between mean and median.

Take Thomson Higher Education's workshop on Central Tendency & Variability.

Just in case you didn't notice: Why is the variable's level of measurement an important consideration when selecting an appropriate measure? When do we use N-1 instead of N as a denominator when computing for certain measures of variability?

The measure of skewness (or asymmetry) is also used to describe data although it is less often reported in published psychological research reports.

• view an interactive illustration of this concept
• the relationship of the measures of central tendency in symmetric and in skewed distributions

The measure of kurtosis (or flatness/peakedness) is the last of the four sets of measures; this is almost never reported by researchers in psychology.

You must have read that median, one of the three most often used measures of central location, is also known as 50th percentile (%ile). Also, one measure of variability is computed by getting the distance between the first and third quartiles. Quartile is a special name for some specific percentiles. Many standardized test scores are expressed in %iles. Click here if you wish to review this topic.

Note: Although you may have learned in your Elementary Statistics course that the four sets of measures may be calculated for grouped data, for our purposes we will be computing for them directly from raw data because

• doing such will yield more accurate values and
• softwares that can process large amount of data will be available for our use.

Below are PPt presentations from the publishers of our book:

Click HERE and HERE (password protected) if you wish to view the downloadable handouts with notes.

Review the lesson using the the following materials from Wadsworth Cengage:

• crossword puzzles: 1, 2
• flashcards: 1, 2
• tutorial quizzes: 1, 2
  

Take Self Tests 03 and 04 now.