(e.g., click-through rate for a banner advertisement). As the name implies, two versions (A and B) are compared, which are identical except for one variation that might impact a user's behavior. Version A might be the one currently used, while version B is modified in some respect. For instance, on an e-commerce website the purchase funnel is typically a good candidate for A/B testing, as even marginal improvements in drop-off rates can represent a significant gain in sales. Significant improvements can be seen through testing elements like copy text, layouts, images and colors. Multivariate testing or bucket testing is similar to A/B testing but tests more than two versions at the same time. Number of participants In the early 1990s, Jakob Nielsen, at that time a researcher at Sun Microsystems, popularized the concept of using numerous small usability tests—typically with only five participants each—at various stages of the development process. His argument is that, once it is found that two or three people are totally confused by the home page, little is gained by watching more people suffer through the same flawed design. "Elaborate usability tests are a waste of resources. The best results come from testing no more than five users and running as many small tests as you can afford." The claim of "Five users is enough" was later described by a mathematical model which states for the proportion of uncovered problems U where p is the probability of one subject identifying a specific problem and n the number of subjects (or test sessions). This model shows up as an asymptotic graph towards the number of real existing problems (see figure below). In later research Nielsen's claim has been questioned using both empirical evidence and more advanced mathematical models. Two key challenges to this assertion are: Since usability is related to the specific set of users, such a small sample size is unlikely to be representative of the total population so the data from such a small sample is more likely to reflect the sample group than the population they may represent 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. It is worth noting that Nielsen does not advocate stopping after a single test with five users; his point is that testing with five