The upper quartile (Q3), or third-quartile, of a data set, is the data value three-quarters, or 75% from the start once the data has been ordered. The position of the third-quartile, in a small data set of n values, is found by finding three-quarters of (n + 1). For larger data sets of size n, often when the data has been summarised, and we are only estimating, the position of the upper quartile is found by finding three-quarters of n.
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