Measures of Central Tendency
o Add up all scores and divide by number of scores.
Mean is sensitive to the exact value of all the scores in the
The sum of the deviations about the mean equals zero. Written
out: ∑ ̅
o The mean is sensitive to extreme scores.
o The sum of the squared deviations of all the scores about their
mean is a minimum. Written out: ∑ ̅ is a minimum.
o Under most circumstances, of the measures used for central
tendency, the mean is least subject to sampling variation.
The Overall Mean
Used to calculate the mean of all scores when presented with
the mean of several different groups.
o Ex. There are three groups; the first includes 50 subjects with a
mean of 75, the second includes 40 subjects with a mean of 80,
and the third includes 25 subjects with a mean of 70. What is
the overall mean?
The Median (Mdn): the scale below which 50% of the scores fall. It
is therefore the same calculation as P .50
o Less sensitive than the mean to extreme scores.
o Under usual circumstances, the median is more subject to