Assume that local average signal strength field measurements were made inside a building, and post processing revealed that the measured data fit a distance-dependent mean power law model having a log-normal distribution about the mean. Assume the mean power law was found to be Pr(d) ∝ d-3. If a signal of 1 mW was received at d0 = 1 m from the transmitter, and at a distance of 10 m, 10% of the measurements were stronger than –25 dBm, define the standard deviation, σ, for the path loss model at d = 10m.