The pureness of a product is checked at regular intervals by the chemical analysis of a sample from each batch.
Analyses for nitrogen content, averaged over a long period, had given a figure of 4.02% with a standard deviation of 0.06%
Then samples taken from 4 successive batches were found to have a mean nitrogen of 4.14%.
Does this indicate a significant change in the composition of the product at 5%? or at the 1% level?
Is choosing a level of significance at the outset a matter of deciding the risk one is prepared to take concluding that the composition of the product has change when in reality it hasn't?
Thanks for any help.