The probability of a defect when defects are correlated. For example, when linewidths are printed too wide the process can cause thousands of ‘bridging’ defects, so although the number of defects is extremely high, there is only one opportunity. Unless of course there are say four sensitive areas on the circuit so that a slight ‘under exposure’ condition would only cause say four sensitive areas to bridge, in which case Zadj would then have four opportunties.