9.7.3 To ensure the ongoing quality of results delivered by a measurement proces

Question

9.7.3 To ensure the ongoing quality of results delivered by a measurement process, an analytical laboratory measures the purity of a standard each time for he most recent five assays are provided in Table 9.7. We know the following about the past evaluation of the measurement process: Previous moving ranges of the purity of the standard have been in control with MR =.70.

The standard used is an internally developed one, so the purity of the standard cannot be considered "known." However, the purity of the standard is thought to remain stable across time. In the past the individual chart of determinations on this standard have been in control with a centerline of 95.8.

Using the data provided in table 9.7, answer the following questiuons:

A. Has the short-term variation of the measurement process used to check for purity remained consistent?

B. Has the measurement process maintained a consistent average?

C. What, if anything, is known about the accuracy of the measurement process?

D. If appropriate, estimate the measurement variation for the measurement process used to evaluate purity.

Table 9.7 Purity Measurements of Standard

Detrmination                           Purity Reading

        1                                                96.2

        2                                                96.5

        3                                                95.3

        4                                                96.0

        5                                                95.5

Explanation / Answer

Generally, the short-term variation of the measurement process cannot be used to check for purity remained consistent.

The sample size (n) can then be expressed as the largest integer less than or equal to =

n = 0.25/SE2

SE = stddev/sqrt(n) = 0.4949/sqrt(5)= 0.2213

n = 0.25/0.2213^2= 5.10

So, in this case appx 5 observations are sufficient.

b) Mean = (given) = 95.8

UCL = X+A2 * R

= 95.8 + 0.577*1.2

= 96,49

LCL = X-A2*R

= 95.8-0.577*1.2

= 95.10

(A2 - value from X and R chart), R - range

except for one observation, process is consistent.

C) Accuracy for 5 observations would be less. For more accurate results the same experiment can be repeated multiple times.

D) Variation of measurement process

= standard deviation ^ 2

= 0.4949^2 = 0.2449

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