Gentlemen:
It is the practice in the pharmaceutical industry to establish the expiration period of drug products by regressing the characteristic of interest with time to find the time where the characteristic no longer falls within specification.
One characteristic of interest for drug products where the active ingredient is not fixed in position as in a tablet or capsule, but is mobile as in an ointment or creme where diffusion is possible.
When the active ingredient diffuses, its homogeneous distribution throughout the mass is altered and assays of product segments taken from locations throughout the mass begin to substantially differ from one another. Homogeneity is measured by Relative Standard Deviation of the assays from various locations in the mass. And there is an upper limit to RSD beyond which the product is considered failed.
I have done this and find very good empirical fits to data by various indicators including normal probability plots of residuals, lack-of-fit test, and other indicators.
I have no theoretical bases for these fits.
I wonder if anyone has assessed RSD vs time in situations like these.
All comments are welcome.
Thank you.
Stan
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