Page 35 - Quality control of pharmaceuticals (07-PA 704)

The subscript (n - 1) indicates that t depends on this quantity, which is

known as the number of degrees of freedom, d.f. (usually given the

symbol ν). The term ‘degrees of freedom’ refers to the number of

independent deviations which are used in calculating s. The

value of t also depends on the degree of confidence required. Some

values of t are given in Table 2.

A more complete version of this table is given in Table A.2 in Appendix

2.

For large n, the values of tn-1 for confidence intervals of 95% and 99%

respectively are very close to the values 1.96 and 2.58 used in

Example 1. The following example illustrates the use of Eq. (2).

Students dissolved zinc from a galvanized nail and measured the mass lost
by the nail to tell how much of the nail was zinc. Here are 12 results:

Mass loss (%): 10.2, 10.8, 11.6, 9.9, 9.4, 7.8, 10.0, 9.2, 11.3, 9.5, 10.6,
11.6

The value 7.8 appears out of line. A datum that is far from other points is
called an outlier.