Page 167 - DMGT501_OPERATIONS_MANAGEMENT

Unit 6: Quality Assurance and Control

Notes
Figure 6.1: Process Capability

-3 +3
X
6

Procedure for Evaluation
This involves the following steps:
1. Collect data on a number of rational groups and review them for consistency or
homogeneity.
2. Eliminated data which does not conform to the general pattern observed. If the data gets
eliminated by more than 1/3rd, then reject the entire data and collect fresh data till
homogeneity is achieved.
3. Calculate the process capability as given below: Calculation of Process Capability Attribute
data
(a) Uniform Sample Size, n = number of observations in each sample.
(i) Count the number of samples say k = 25,
(ii) Let sample size (number of observations in each sample) = n,
(iii) Add all the defectives observed in all the 25 samples, summation of defects =d,
(iv) Find m = np = D/k where ‘p’ is the proportion defective,
(v) For p < 0.10, read limits from the concerned statistical table, against the value
of ‘m’,
Else (m >= 0.10),

Calculate limits as np 3 n.p.q , where q = (1 – p),

(vi) Test for homogeneity: See whether all readings are within Control Limits, if so,
accept the data as homogeneous. Otherwise, reject the readings (observations)
which are outside control limits and again test for homogeneity. If the total
number of observations rejected are more than one-third reject the entire data
and fresh data should be collected.
(b) Variable sample size (number of observations in each sample not uniform)
(i) Let number of samples = k

(ii) Let n , n … n be the sample sizes (n + n + … + n = N)
1 2 k 1 2 k
(iii) Let d , d … d be the number of defectives found in corresponding samples (d
1 2 k 1
+ d + … + d = D)
2 k

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