· Fit a linear regression of content (or critical quality attribute) over time, and compute the 95% one-sided confidence limit.

· Shelf life = the time at which the confidence limit crosses the specification limit: if content decreases over time, look at the lower bound crossing LSL; if it increases, look at the upper bound crossing USL.

· If the intersection lies well outside the regression range and beyond 3× the tested duration, it is not suitable for real batch extrapolation and is for reference only.