The average total inspection per lot depends on the incoming quality, the probability that the lot will be accepted, and the sample and lot sizes. When incoming quality is very good (for example, no product in the lot has a defect), then you inspect the specified sample size. When the incoming quality is very bad (for example, every product in the lot has a defect), then you resort to 100% inspection. Because the quality levels varies between lots, the average number inspected across many lots falls between 0% and 100% of the lot size.
The average total inspection (ATI) plot depicts the relationship between the quality of the incoming material and the number of items that need to be inspected, assuming that rejected lots will be 100% inspected and that a rectifying inspection of defective items will be performed. In a rectifying inspection, defective items are either removed, reworked, or replaced.
In addition to the ATI curve, you should also examine the OC curves and the AOQ curves when rectifying inspection is required.