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Multiattribute acceptance sampling plans - Library(ISI Kolkata ...

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(i) Poisson as approximation to binomial and multinomial<br />

If p i → 0, n → ∞ and np i → m i then the binomial probability b(x i , n, p i ) tends to<br />

Poisson probability g(x i , np i ) where<br />

( ) n<br />

x i<br />

and<br />

b(x i , n, p i ) =<br />

p x i<br />

i (1 − p i ) n−x i<br />

g(x i , np i ) = e −np i<br />

(np i ) x i<br />

/(x i )!.<br />

Under this conditions for i = 1, 2, ..., r, the expression given as equation (1.1.3) can be<br />

modified as<br />

r∏<br />

P r(x 1 , x 2 , ..., x r | p 1 , p 2 , ..., p r ) = g(x i , np i ); x i ≥ 0, i = 1, 2, ..., r.<br />

i=1<br />

...(1.1.7)<br />

r∑<br />

If we also make an additional assumption that p i → 0 then the equation (1.1.6) can also<br />

i=1<br />

be modified as (1.1.7).<br />

(ii) Poisson as an exact distribution and occurrences of defect types independent<br />

In case we count the number of defects per item for each characteristic, we may construct a<br />

model for which the expected number of defects for a item for i th the characteristic equals<br />

to p i in the long run. We assume the number of defects for r distinct characteristics in a item<br />

are independently distributed. The output of such a process is called a product of quality<br />

(p 1 , p 2 , ..., p r ), the parameter vector representing the mean occurrence rates (of defects) per<br />

observational unit. The total number of defects for any characteristic in a lot of size N from<br />

such a process will vary at random according to a Poisson law with parameter Np i for the i<br />

th characteristic under usual circumstances.<br />

Similarly, the distribution of number of defects on attribute i, in a random sample of size<br />

n drawn from a typical lot will be be a Poisson variable with parameter np i . Independence<br />

of the different characteristics will be naturally maintained in the sample, so that the joint<br />

probability of occurrence can be expressed as (1.1.7).<br />

Here p i , i = 1, 2, ..., r denotes the average number of defects per item in respect of characteristics<br />

i instead of proportion defectives, since in this situation we are dealing with defects<br />

rather than defectives.<br />

38

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