Variability of Data
nRange
nhighest score minus the lowest score
nOutliers will skew your data
nInterquartile Range Ð 25th, 50th, 75th, 90th, etc 25th quartile Ð 25% of all scores fall below this score
n 75th quartile Ð 75% of all scores fall below this score
nSemi-interquartile Ð Q3ÐQ1   (middle 50%) Ð not affected by outliers
n                       2
nThese are CRUDE MEASURES OF VARIABILITY - does not tell the whole story about the raw data. A mean of 5.3 does not tell if most of the scores were similar. So, we have measures of variability, which tell us about the spread of the scores on the scale of measurement.
nDeviation
nthe distance from the mean
n+ is above mean, - is below the mean
nDeviation scores - MUST sum (S) to zero