I have had many people ask me how I come up with my ratings. Even since I started the blog I have changed around my system. The short answer is: it's somewhat arbitrary, it is based upon recent beers I've had of the same style, and reflects my personal tastes and not any others.
When I first started sampling beers, I rated things 1-5, with whole numbers only. This was because I kept an excel spreadsheet of every beer I've ever had (and still do-let me know if you'd like it emailed to you) and the whole numbers allow for easier tracking. When I started the blog, I figured I would allow for fractional numbers in the 1-5 system. This matches what many ratings sites do. They will allow you to rate appearance, aroma, mouthfeel, taste and overall. Each of these are on a scale of 1-5. I like this scale, but then people would say to me "wow-you have this a 4/5 but beer advocate gave it a 98. Was it not good?" This encouraged me to change my system to more reflect Beer Advocate's.
Their rating system tends to more reflect a letter grade scale, with the majority of beers falling in the 70-100 range. I have rarely seen anything below a 50. With exceptional beers, I put all of those in the "A" category. From 90-100, there is not much room to distinguish one exceptional beer from another. In my opinion, anything 80 and above is a great beer, though I still buy 70 and above beers. I look at 75 being an "average" rating.
So how do I actually pick the final number? It really is somewhat arbitrary. I figure out which bucket it fits into, and then think about the last few beers of a similar style and overall that I have rated. If it is better, I try to make the number reflect that. If it is indistinguishable or worse, then the number will reflect that.
Additionally, people have wondered why so many beers are 95 and above. The answer is that I tend to buy many highly acclaimed and regarded beers. These often warrant world class status. If I sampled things like Bud Light, Corona or PBR, I'm sure there would be some 60s thrown into the mix. In my first few months I have been trying different and exciting beers, though once I begin exhausting the local store I will move on to more common ones!
One final point is that just because something scores a 95 last month, something I may rate a 94 might be better in my book. My tastes evolve over time, though it is way too much time and energy to be constantly reevaluating them. For example, in college I would have rated Boston Lager a 100 since it was vastly different than Natural Light or Pabst. However, I look back on that as a beer in the mid 80s. The same happens the first few times a try a new style or hard to find brand. (I think Yuengling before it came to Boston or NY-I look at that as an average beer now). Tastes certainly do change, and my ratings must evolve with it. It will be interesting next year when some of the seasonals I have rated come out and I can see the differences in my ratings.
Hope this clears some things up. I would love, as always to hear comments about your rating scale.