1. A single-look, unlimited-reply sequential probability ratio test device SLUR-SPRT for use in a Friend Identification system, said Friend Identification system having the following inputs available to said SLUR-SPRT device: T.sub.C - upper threshold number, T.sub.B - lower threshold number, R.sub.O - a-priori probability ratio, p(a/f)-probability that an acceptable reply will be received in response to any particular interrogation selected at random from those sent to a friend, p(a/f) probability that an acceptable reply will be received in response to any particular interrogation selected at random from those sent to an enemy, LE-leading edge pulse, TE-trailing edge pulse, (a)-acceptable reply pulse, (a) - unacceptable reply pulse, M-interrogation trigger pulses, K-a number equal to or slightly greater than the average number of missed replies required by a trailing edge detector to produce a trailing edge declaration, comprising: threshold register means for receiving, storing, and providing as outputs said upper threshold number T.sub.C and said lower threshold number T.sub.B ; ratio product determiner means receiving said inputs R.sub.O, p(a/f), p(a/f), LE, TE, (a), (a) for producing a ratio product R.sub.n after each said reply has been evaluated and providing R.sub.n as an output, and additionally providing a delayed reset signal as an output; comparator means for receiving and comparing said ratio product R.sub.n and said inputs T.sub.B and T.sub.C from said threshold register to determine if R.sub.n is greater than or equal to T.sub.C, less than or equal to T.sub.B or between T.sub.C and T.sub.B and provide an indication of said determination as an output; Slur output selector means receiving said input K, said output of said comparator means, said M input and said delayed reset signal from said ratio product determiner means, for selecting for Mode 1 operation: an output of accept A, reject R, or unknown U; or for Mode 2 operation: an output of accept A, or not acceptable A; and for either mode of operation, a reset bus pulse to be sent back to said ratio product determiner.