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

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2.1.5 Approximation under Poisson conditions<br />

As discussed in chapter 1.1 we use the phrase ‘Poisson conditions’ when Poisson probability<br />

can be used in the expressions of type B OC function in the two situations.<br />

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 />

g(x i , np i ) = e −np i<br />

(np i ) x i<br />

/(x i )!.<br />

Under this condition the expression P (p) given in equation (2.1.5) can be modified as :<br />

P (p) =<br />

∑<br />

r∏<br />

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

x 1 ,x 2 ,...,x r∈A<br />

i=1<br />

...(2.1.9)<br />

r∑<br />

If we also make an additional assumption that p i → 0 then the expression P (p) given in<br />

i=1<br />

equation (2.1.8) can also be modified as (2.1.9).<br />

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

We assume the number of defects for r distinct characteristics in a unit are independently<br />

distributed. The output of such a process is called a product of quality (p 1 , p 2 , ..., p r ), the<br />

parameter vector representing the mean occurrence rates of defects per observational unit.<br />

The total number of defects for any characteristic in a lot of size N drawn from such a<br />

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

characteristics under usual circumstances. Similarly the distribution of number of defects<br />

on attribute i, in a random sample of size n drawn from a typical lot will be a Poisson<br />

variable with parameter np i . Independence of the different characteristics will be naturally<br />

maintained in the sample so that the type B probability of <strong>acceptance</strong> will be given by the<br />

expression of P (p) given as equation (2.1.9). In this situation we will be dealing with defects<br />

rather than defectives.<br />

2.1.6 The genaralized cost model<br />

Thus under Poisson condition described as above the expression of K(N, n, p) is same as<br />

(2.1.5) with<br />

P (p) = ∑ s∈A<br />

∏ ri=1<br />

g(x i , np i ) and Q(p) = 1 − P (p)<br />

84

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