fixed value is the smallest possible value if the monotonic sequences are decreasing sequences vice versa 0047 fig7 illustrates 3 curves for representing 3 mean monotonic sequences in fig7 curves 30 and 34 have no step variation therein however curve 32 has a step variation in a medial position therefore the tool corresponding to the curve 32 can be determined to have chamber inconsistency curves 30 and 34 are very similar to an exponential curve such as ae u2212 bx which is a non match for curve 32 therefore the tool corresponding to curve 32 can be determined to have chamber inconsistency according to its incompatibility with the mathematical equation the medial position refers to the position not located within the first several elements or the last several elements the detail explanation follows if for example most of the retrieved lots have 25 wafers in a lot and the tool with chamber inconsistency has a total of two chambers but one inconsistent chamber therein and the location of the step variation of the mean monotonic sequence should be around the 13 th 25 1 2 element by the same theory if most of the retrieved lots have 25 wafers per lot and the tool with chamber inconsistency has a total of five chambers the location of the step variation of the mean monotonic sequence should be around the 5 th 10 th 15 th or 20 th respectively corresponding to the total number 1 2.