Skewed distribution

Hello all!

I have a sample of 30 values from a production process that should be normally distributed. In fact, it shows a distribution heavy on the left. Any idea why?


TS Contributor
Some possibilities:
a) your assumption that it should be distributed normally, was wrong.
b) a rare instance of chance variation; even with n=30, some samples from a normally distributed population can be skewed.
c) the influence of an unanticipated second factor.
Thank you for the quick reply.

a) In a stable production process with millions of parts passing through, for example drilling the same hole. If I take a random sample, shouldnt it be normally distributed?


TS Contributor
A sample of n=30 is never perfectly normally distributed, even if the underlying distribution is normal.
Most of the time, the samples will a show a distribution quite close to a normal distribution. But due to
chance, a certain proportion of samples will look different.
I know that n=30 is very small and I can not expect a perfect normla distribution, but this sample looks really off. I will have the guys do deeper digging on this topic.



Less is more. Stay pure. Stay poor.
Do you have historic data to compare it too?

Can you post the data in a histogram?

What is the actual outcome variable (e.g., time, size, etc.)?

Have you put precision bounds on it to show that it is outside an expected range for a stable process?

@Miner please help direct them on how to do this or given any relevant advice!