When examining process data, it is possible to work at either lot (batch) or wafer level. Which should we choose?
There is no simple answer to this question. Suppose we have six lots, three of which are bad, and three of which are good. Analyst A works at the batch level and concludes that there is not enough data for a statistically significant result. Indeed, Q-YIELD warned of this possibility when importing such a small amount of data. Analyst B works at the wafer level and projects some batch measurements onto each wafer record. We now have 24*6 = 144 wafer records. Analyst B finds a relationship which can explain why 72 of these 144 wafers have a high defect density. This relationship is statistically highly significant. In one sense nothing is different: in each case we processed 6 batches of 24 wafers. The question we have to ask ourselves is this: is the evidence from each wafer statistically independent? i.e. was the information used to draw our conclusions based upon the same observation or a different observation. If the information was obtained by aggregating data batch and then projec
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