SOP on Sampling Plans

  

Sampling Plans

(Scale of  Sampling )

1.0  Purpose: This SOP describes a procedure to draw a
sample by selecting sampling units.

2.0  Scope: This SOP is applicable to all the vendors or
the suppliers with a high confidence level.

3.0  Responsibility: Quality Control Chemist to ensure that
the procedure is followed
 

4.0  Materials and Equipment: None

5.0  Procedure: The object of the sampling is to find out
the quality of materials at different points in a container. The consignments
received from approved suppliers with a high confidence level may be sampled by
selecting sampling units on random basis. For random sampling, each sampling
unit is consecutively numbered and required number of sampling units depends on
the following assumptions –

     5.01 The
“n plan”:
In this plan, samples can be withdrawn from any part of the
container

             (Usually
from the top layer). It is based on the formula:

n
=
Ö
N

                                 Where, N=
number of sampling units in the consignment.

            The
value of n is rounded off to the next higher integer. According to this plan,
original samples are taken from n sampling units selected at random and they
are placed in separate sample containers. Each sample is examined for
appearance and identity test according to the specifications. If the result on
these samples are concordant, the original samples are pooled into final sample
from which analytical sample is prepared and is tested for complete analysis.
The remaining sample is kept as retention sample. The “n plan” should be used
only when the material is considered uniform (e.g. same batch) and is supplied
by a well known supplier.

 

5.02 The “p plan”:
This plan is used when the material
is uniform and has been received from a well known & time tested supplier.
It may be used when the main aim is to check identity. This plan is based on
the formula –

p = 0.4Ö N

                          Where, N= number of
sampling units in the consignment

            Under
this plan, samples are taken from each of the N sampling units of the
consignment and are placed in separate sample containers. These samples are
individually examined for appearance and identity. If the result of samples are
concordant, p final sample, are formed by appropriate pooling of the original
samples.

 

5.03 The “r plan”:
 This plan may be used when the
material is either suspected to be non-uniform or has been received from a
supplier who is not well known. This plan is based on the formula:

r =
1.5
Ö
N

                          Where, N= number of
sampling units in the consignment

            Under this plan, samples are taken
from each of the N sampling units of the consignments. The samples are
collected in separate containers. All the samples are tested individually for
identification as per specification. If the results of testing are concordant,
r samples are randomly selected and individually subjected to complete testing.
If the result of r samples are concordant, pooled sample may be kept at
retention sample.

 Table – Values of n, p or  r for N sampling units

Value of n, p or r

Values
of  N

n plan

p plan

r plan

2

Upto
4

Upto 25

Upto 2

3

5-9

26-56

3-4

4

10-16

57-100

5-7

5

17-25

101-156

8-11

6

26-36

157-225

12-16

7

37-49

 

17-22

8

50-64

 

23-28

9

65-81

 

29-36

10

82-100

 

37-44

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