Statistical techniques are used to derive closed-form. approximate solutions for estimating a standard deviation on the fracture stress determined from mirror size measurements. The estimated coefficient of variation of fracture stress is noted to be twice as sensitive to the variation of the mirror constant, A(j), as to the variation in the measured boundary radius. r(j). This allows the hackle radius to be reasonably estimated and bounded from the mirror and branch radii when it cannot be clearly delineated.