Below is one example that Ben Gomes provides, can you spot the difference?
In this case, the difference in user interaction is so clear and marked we could tell extremely quickly which one worked better: the difference is in the thickness of the plus box next to the stock quote. Now, coming to the conclusion that one is “better” is tricky, and there’s many a possible slip on the way there. Does more interaction with the plus box mean that it is better? How about if users then miss good results because they are distracted by the more prominent plus box? If we’ve done our job right, almost without your noticing, things will work just that little bit better for you.