506
ANALYTICAL CHEMISTRY
bility of results. is :
The usual formula for standard error of means
S.F..W.
=
2/Z:X2/A’(N - 1)
where Z X 2 is the sum of the squares of deviations of individual values from the mean and N is the number of individual values. Use of standard error of means is invaluable in showing whether the difference between the scores of two samples is significant. In general, if two scores differ by the sum of their standard errors, this assumption may be taken as practically a certainty. In order to illustrate the type of results obtained on scoring various samples using 100 as a perfect score, a series of scores on samples oi varying quality is listed. This illustrates the accuracy of scores and shows that the accuracy of results decreases as the total scores decrease. This pattern of results often occurs when therr IS no conceivable Kay of illustrating a very poor sample ~it: hiscore of 0. Sarnpl,.
A
B C
n
E: f
6;
H
No. of Scores 11 13 12
14
15 15
12 18
Scores -.___~~_ ____ RIaxirnum 100 0
