Quadratic mean diameter
In forestry, quadratic mean diameter or QMD is a measure of central tendency which is considered more appropriate than arithmetic mean for characterizing the group of trees which have been measured. For "n" trees, QMD is calculated using the quadratic mean formula: formula_1 where formula_2 is the diameter at breast height of the "i" tree. Compared to the arithmetic mean, QMD assigns greater weight to larger trees – QMD is always greater than or equal to arithmetic mean for a given set of trees. QMD can be used in timber cruises to estimate the standing volume of timber  in a forest, because it has the practical advantage of being directly related to basal area, which in turn is directly related to volume. QMD can also be calculated as: formula_3 where BA is stand basal area, n is the number of trees, and k is a constant based on measurement units - for BA in ft and DBH in inches, k=0.005454; for BA in m and DBH in cm, k=0.0000785.