EPE is Estimated Positional Error (horizontal)
EPE = HDOP * URA (1sigma) would give a measure of the 68%
confidence circle, i.e., 68% of your position fixes would fall
within and 32% would fall outside.
EPE = HDOP * URA (1sigma) * 2 gives a measure of the 95%
confidence circle, i.e., 95% of your position fixes would fall
within and 5% would fall outside.
EPE = HDOP * URA (1sigma) * 0.73 gives a measure of the 50%
confidence circle, i.e., 50% of your position fixes would fall
within and 50% would fall outside. This is also known in the
literature as CEP (Circular Error Probable).
Definitions:
 HDOP  Horizontal Dilution of Precision
 URA  User Range Accuracy is a quantity that is transmitted in the navigation message that is the predicted (not
measured) statistical ranging accuracy. Since it is defined for SPS (Standard Positioning Service), it includes SA.
