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If we use central differences rather than the forward difference of the Euler method or the backward difference of the backward Euler , we may obtain a second order method due to cancellation of the terms of O ( Dt2 ) . In other words , the key difference between the old advertising model and the Google model is this : advertising on search engines allows companies to deliver context-based ads to people who have already expressed an interest ( by typing in a specific keyword ) in a particular topic . The experimentalist however is less concerned with sample sizes and typically assumes relative homogeneity among subjects of whatever ability is required to perform a task , often relegating inter-subject differences into the category of error variance . The key difference between the two technologies is that while virtual reality creates a completely illusory environment , augmented reality supplements the real world - - through a head-mounted display , for example - - with information intended to make a particular task easier . On the other side , in the " Glaubenslehre , " an inner difference in the types of faith , whereby the communions are divided , is maintained . A key difference between the Bible and say the Koran is that the Bible is not believed by christians to have been directly composed by God . As was pre - viously explained in more detail on page 7 of the Audit Activities section , the key difference between the two cate - gories stems from how premium rates are calculated for each . The key difference between the decor at , say , Quaker Steak and Lube and the one at the Double Wide is that Quaker Steak pretends to be a garage , while Double Wide actually was one . The hypothesis that differences in rates of maternal depression could be associated with group-level differences in the attainments of children , however , has not been directly tested , so it is necessary to go through each link in the causal chain .