In coal testing, a parameter, B, applied to the ash curve: B = (p75 - p25)/2(p50 - 1), where pX is the specific gravity of particles of which the fraction separated is X percent, (p75 - p25) is the statistical intermediate or inquartile range, and p50 is the effective 287 density of separation in a process in which a dense medium, vertical current, or jigging action is used. Equation is used to define shape of a Tromp curve.