probable error

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The amount by which the arithmetic mean of a sample is expected to vary because of chance alone.

GNU Webster's 1913

1. n. (Mensuration) See under Probable.
2. adj. (of an observation, or of the mean of a number), that within which, taken positively and negatively, there is an even chance that the real error shall lie. Thus, if 3″ is the probable error in a given case, the chances that the real error is greater than 3″ are equal to the chances that it is less. The probable error is computed from the observations made, and is used to express their degree of accuracy.