The lower and upper quartiles (this term was first used by Galton, 1882; also referred to as the .25 and .75 quantiles) are the 25th and 75th percentiles of the distribution (respectively).

The 25th percentile of a variable is a value such that 25% of the values of the variable fall below that value. Similarly, the 75th percentile is a value such that 75% of the values of the variable fall below that value and is calculated accordingly.

You can specify the method of computing percentiles in the Options dialog box - Analyses/Graphs: Limits tab (accessible by clicking Options on the Tools tab or menu). For more information, see Percentile Calculation Options in STATISTICA. User-specified percentiles can be calculated in Nonparametrics.