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530<br />

Part Three<br />

Planning and control<br />

Table S17.2 Factors for the calculation of control limits<br />

Sample size n A 2 D 3 D 4<br />

2 1.880 0 3.267<br />

3 1.023 0 2.575<br />

4 0.729 0 2.282<br />

5 0.577 0 2.115<br />

6 0.483 0 2.004<br />

7 0.419 0.076 1.924<br />

8 0.373 0.136 1.864<br />

9 0.337 0.184 1.816<br />

10 0.308 0.223 1.777<br />

12 0.266 0.284 1.716<br />

14 0.235 0.329 1.671<br />

16 0.212 0.364 1.636<br />

18 0.194 0.392 1.608<br />

20 0.180 0.414 1.586<br />

22 0.167 0.434 1.566<br />

24 0.157 0.452 1.548<br />

Worked example<br />

GAM (Groupe As Maquillage) is a contract cosmetics company, based in France but with<br />

plants around Europe, which manufactures and packs cosmetics and perfumes for other<br />

companies. One of its plants, in Ireland, operates a filling line which automatically fills<br />

plastic bottles with skin cream and seals the bottles with a screw-top cap. The tightness<br />

with which the screw-top cap is fixed is an important part of the quality of the filling line<br />

process. If the cap is screwed on too tightly, there is a danger that it will crack; if screwed<br />

on too loosely it might come loose when packed. Either outcome could cause leakage<br />

of the product during its journey between the factory and the customer. The Irish plant<br />

had received some complaints of product leakage which it suspected was caused by<br />

inconsistent fixing of the screw-top caps on its filling line. The ‘tightness’ of the screw<br />

tops could be measured by a simple test device which recorded the amount of turning<br />

force (torque) that was required to unfasten the tops. The company decided to take<br />

samples of the bottles coming out of the filling-line process, test them for their unfastening<br />

torque and plot the results on a control chart. Several samples of four bottles were<br />

taken during a period when the process was regarded as being in control. The following<br />

data were calculated from this exercise:<br />

The grand average of all samples D = 812 g/cm 3<br />

The average range of the sample A = 6 g/cm 3<br />

Control limits for the means (B) chart were calculated as follows:<br />

UCL = D + A 2 A<br />

= 812 + (A 2 × 6)<br />

From Table 17.6, we know, for a sample size of four, A 2 = 0.729. Thus:<br />

UCL = 812 + (0.729 × 6)<br />

= 816.37<br />

LCL = D − (A 2 A)<br />

= 812 – (0.729 × 6)<br />

= 807.63

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