<Li> Not every usability problem is equally easy - to - detect . Intractable problems happen to decelerate the overall process . Under these circumstances the progress of the process is much shallower than predicted by the Nielsen / Landauer formula . </Li> <P> It is worth noting that Nielsen does not advocate stopping after a single test with five users; his point is that testing with five users, fixing the problems they uncover, and then testing the revised site with five different users is a better use of limited resources than running a single usability test with 10 users . In practice, the tests are run once or twice per week during the entire development cycle, using three to five test subjects per round, and with the results delivered within 24 hours to the designers . The number of users actually tested over the course of the project can thus easily reach 50 to 100 people . </P> <P> In the early stage, when users are most likely to immediately encounter problems that stop them in their tracks, almost anyone of normal intelligence can be used as a test subject . In stage two, testers will recruit test subjects across a broad spectrum of abilities . For example, in one study, experienced users showed no problem using any design, from the first to the last, while naive user and self - identified power users both failed repeatedly . Later on, as the design smooths out, users should be recruited from the target population . </P> <P> When the method is applied to a sufficient number of people over the course of a project, the objections raised above become addressed: The sample size ceases to be small and usability problems that arise with only occasional users are found . The value of the method lies in the fact that specific design problems, once encountered, are never seen again because they are immediately eliminated, while the parts that appear successful are tested over and over . While it's true that the initial problems in the design may be tested by only five users, when the method is properly applied, the parts of the design that worked in that initial test will go on to be tested by 50 to 100 people . </P>

According to your text the real issue in testing is