The interquartile range is a measure of spread used most commonly with the median. And the cut-off for the fourth quartile is the 100 th percentile, which is the maximum. The cut-off for the third quartile is the 75 th percentile. The cut-off for the second quartile is the 50 th percentile, which is the median. The cut-off for the first quartile is the 25 th percentile. Sometimes, epidemiologists group data into four equal parts, or quartiles. The maximum value is the 100 th percentile, because all values fall at or below the maximum. The median, the halfway point of the distribution, is the 50 th percentile. In other words, the 90 th percentile has 90% of the observations at or below it. The Pth percentile (P ranging from 0 to 100) is the value that has P percent of the observations falling at or below it. Percentiles divide the data in a distribution into 100 equal parts. Statistically, subtract the minimum from the maximum value.įor an epidemiologic or lay audience, you could report that “incubation periods ranged from 15 to 31 days.” Statistically, that range is 16 days. Epidemiologically, report the minimum and maximum values. Identify the smallest (minimum) observation and the largest (maximum) observation. In the epidemiologic community, the range is usually reported as “from (the minimum) to (the maximum),” that is, as two numbers rather than one. In the statistical world, the range is reported as a single number and is the result of subtracting the maximum from the minimum value. The range of a set of data is the difference between its largest (maximum) value and its smallest (minimum) value. Measures of spread include the range, interquartile range, and standard deviation. Just as measures of central location describe where the peak is located, measures of spread describe the dispersion (or variation) of values from that peak in the distribution. Spread, or dispersion, is the second important feature of frequency distributions.
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