The chart of S/NT shown in Figure 2 uses a value of 50 for the target; the two charts are very similar. The S/NT value on the upper limit of 39.3 has a volume precisely on the target and a small standard deviation of 0.542.
The signal-to-noise ratio used by McAllister is set out in the following equation:.
in which T is the target.
For convenience, our laboratory reports of the volume or weight of drug dispensed present a concise summary of the information
on the performance of each batch. This summary consists of the mean, the coefficient of variation, and the minimum and maximum
of 375 individual measurements. This summary provides all the relevant information contained in the individual three replicate
measurements of 125 packs.
The absence of raw data is not a problem because we can calculate S2
T by modifying Equation (1) as follows:
The standard deviation Sx can be estimated by multiplying the coefficient of variation (CV) by the mean or by dividing the range (R) by a constant (d
) that depends on sample size. Values of d
are available in reference books such as Wheeler and Chambers (1992). Some values of d
are: 1.128 for n = 2, 1.693 for n = 3, 3.078 for n = 10 and 3.735 for n = 20. For a sample of 375 (3 repeats x 125 valves), the value of d
Presenting S/NT in the form of Equation 2 may make it easier to understand what the signal-to-noise ratio tells us about the process. We
can see that the term 20 * log10 T, which = 33.979 if T = 50, can be regarded as a baseline.
The second term,