Pocketbook of Useful Formulæ and Memoranda
for Civil and Mechanical Engineers

610 iiolfsworth’s pocket-book Ebbors of Observation. (Paget Higgs.) Let n = number of observations; „ J2 d3 ... d„ = differences from the arithmetical mean; „ S = the sum of the squares of the errors, i. e. S = df + (V + c?32 +..........d-\ Then, the mean error of a single observation /HZ - - r / s ' „ the mean error of the result=± V —7---- ” n(n — 1) the probable error of a single observation / s = ±0-6745 xV • „ the probable error of the result= ± 0'6745 X V n (n - 1)' 0-G745 = -|, for all practical purposes. The probable error means that it is as likely that the actual unknown error is less than the probable error as that it is greater.