Instant Notes: Analytical Chemistry

physical or physico-chemical measurement, but no one specific source can be

identified.

A series of measurements made under the same prescribed conditions and

represented graphically is known as a frequency distribution. The frequency of

occurrence of each experimental value is plotted as a function of the magnitude

of the error or deviationfrom the averageor meanvalue. For analytical data,

the values are often distributed symmetrically about the mean value, the most

common being the normal erroror Gaussian distribution curve. The curve

(Fig. 4) shows that

● small errors are more probable than large ones,

● positive and negative errors are equally probable, and

● the maximum of the curve corresponds to the mean value.

The normal error curve is the basis of a number of statistical tests that can be

applied to analytical data to assess the effects of indeterminate errors, to compare

values and to establish levels of confidence in results (Topics B2 and B3).

24 Section B – Assessment of data

2.5

2

1.5

1

0.5

0

0102030

Size of titer (cm^3 )

Relative error (%)

Fig. 3. Effect of a constant error on titers of differing magnitudes.

Frequency of occurrence

of each deviation

• 0 +
  Deviation from mean, μ

Fig. 4. The normal error or Gaussian distribution curve.