A casual round of beers with friends can be a good learning opportunity. Someone may ask for feedback about her/his beer, or you’re all sampling a particular commercial beer for the first time—you focus your attention and gauge the experience. The caveat is that it’s very subjective. The brewer’s lack of confidence may push you to be overly critical, or what you’ve heard about a given brewery may set your opinion before you’ve even sipped the beer. By contrast, a structured tasting distills that experience into something more objective by peeling away bias and undue influences.
Look at how a product marketing team runs a taste test. They don’t tell people exactly what they’re comparing; they simply prompt them for impressions. The same approach works for beer evaluation, whether the goal is assessing the results of a brewing experiment, training to become a beer judge, or even running a homebrew competition.
These different situations affect the specific steps you should follow, but there are some universal principals to consider.