Page 59 - Pharmaceutical Analytical Chemistry III

•Although correlation coefficients are simple to calculate, they are

all too easily misinterpreted. It must always be borne in mind that

the use of r equation will generate an r-value even if the data are
patently non-linear in character.
•Figure 5.4 shows two examples in which a calculation of r would
be misleading.
•In Figure 5.4(a), the points of the calibration plot clearly lie on a

curve; this curve is sufficiently gentle, although quite a high

correlation coefficient was obtained when r equation is applied.

The lesson of this example is that the calibration curve must

always be plotted (on graph paper or a computer monitor):

otherwise a straight-line relationship might wrongly be deduced

from the calculation of r.
•Figure 5.4(b) is a reminder that a zero correlation coefficient does
not mean that y and x are entirely unrelated; it only means that
they are not linearly related. So we must draw data.