The quartiles of a probability distribution refer to the points (or soemtimes the intervals between them) that divide the paramter into four pieces where the probability of each is exactly a quarter. This means that on average 25% of the time the value will lie below the lower quartile and 25% of the time larger than the upper quartile. Arguably the median can be regarded as the middle quartile, but more often the term is sued only for the lowert and upper quartile.
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Normally distributed data split into four equally probably categories at upper and lower quartiles and median