Complex sentence: The base rate fallacy is only fallacious when non-terrorists outnumber terrorists, or equal them in number; in a city with about 50% terrorists and about 50% nonterrorists, the real probability of misidentification won't be far from the failure rate of the device.
Split into simpler sentences: The base rate fallacy is only fallacious when non-terrorists outnumber terrorists, or conversely. In a city with about 50% terrorists and about 50% nonterrorists, the real probability of misidentification won't be far from the failure rate of the device.