PSYC 218 Chapter Notes - Chapter 5: Quartile, Standard Deviation, Squared Deviations From The Mean
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Measure of variability: a single summary figure that describes the spread of observations within a distribution (range, semi-interquartile range, variance, and standard deviation) Range: the difference between the lowest score and the highest score in a distribution. Ideal for preliminary work or in other circumstances where precision is not an important requirement. Not sensitive to the total condition of the distribution. Semi-interquartile range (q): one-half the distance between the first and third quartile points in a distribution. Quartile points: the three score points the divide a distribution into four parts, each containing an equal number of cases: q1 (=p25, q2 (=p50, q3 (=p75) Properties in common with median because both are defined in terms of percentile points of the distribution. Less sensitive to the presence of a few very extreme scores than is the standard deviation. With open-ended distributions, may be the only measure that is reasonable to compute: impossible to calculate standard deviation for open-ended distributions without making assumptions.
