How do I draw OC curve? How do I plot this curve in Excel?

[vinay_bull]

Lot size = 1500
Sample size = 150
Acceptance no = 3

How Do i plot this curve in Excel?

Also my AQL is 0.05% and LTPD is 6%, how do I describe the protection offered?

[Wesley Richardson]

Originally Posted by vinay_bull

Lot size = 1500
Sample size = 150
Acceptance no = 3

How Do i plot this curve in Excel?

Also my AQL is 0.05% and LTPD is 6%, how do I describe the protection offered?

Hi Vinay,

Use the equation P(r) = ((e^(-np)(np)^r)/(r!)
for your example, n = 150
r = 3
p = lot fraction defective
P(r) = Probability of exactly r defectives in sample of n.

Do this calculation for various values of p, for example over the range from p = 0.005 in 0.005 increments up to p = 0.10. Using the above equation, or built in function in Excel, this can be done quite easily.

Plot the value of P(r) as a function of p.

This is the operating characteristics curve.

Wes R.

[vinay_bull]

Hi,

Thanks for the reply. Any idea about the second question
Also my AQL is 0.05% and LTPD is 6%, how do I describe the protection offered?

[Wesley Richardson]

Originally Posted by vinay_bull

Hi,

Thanks for the reply. Any idea about the second question
Also my AQL is 0.05% and LTPD is 6%, how do I describe the protection offered?

AQL is normally associated with a sampling plan such as ANSI/ASQ Z1.4 for attributes or ANSI/ASQ Z1.9 for variables. The AQL is used to define for a given plan, the accept and reject numbers. Since you have defined your own plan, the AQL is not applicable.

You can determine the AOQ and AOQL from the method I have given above.

Calculate AOQ = (p)(P(r)) for each of the values you found above. AOQL is where AOQ is at the maximum.

One correction for my previous equations, since r = 3, you will also need to calculated P(r=0), P(r=1) and P(r=2), as well as P(r=3) for each p. Add the P values together for a given p to get the total P_a (probability of acceptance) under the OC Curve.

LTPD is lot tolerance percent defective. It is normally associated with Dodge-Romig sampling tables. Common values are LTPD = 5% or 10%. You pick a value, e.g. LTPD = 5%, and then read off the OC Curve the probability of acceptance at that value, P_a.

Wes R.