esaProbeStatsSumOfSqNegR2PJitter is a 64-bit performance counter that reports the sum-of-squares of the negative one-way jitter (packet-to-packet delay variation), in microseconds, from the reflector back to the probe, for the interval that is still being actively collected (the live bin). Negative (a packet arriving earlier than the one before it) jitter from the reflector back to the probe is exactly the kind of packet-to-packet delay variation that causes stutter in voice and video traffic riding over the same service. For example, combining this live sum-of-squares value with the bin's packet count and delay sum lets the technician compute the variance of negative jitter from the reflector back to the probe to judge, right now, whether the circuit is consistently mediocre or wildly erratic.