Achievement Rarity

I'm new to the site, and couldn't find a FAQ (despite clicking FAQ in the Main Menu, which just takes me to the root level of the forums).

I'm not sure what the terms like TSA, that tiny number in () next to the TSA value, and Ratio are:
The Aquatic Jew's Achievements

Also, is there a way to see the Steam Achievement Rarity on the site? For example: https://steamcommunity.com/stats/336140/achievements/

There are achievements that 0.9% of people have gotten; being able to see those percentages on the site would be pretty awesome too. :)

Sorry, I'm new and haven't had any luck searching through the forums to find answers to these questions.

4 years late is better than never, lol.

This article covers what they are in detail, but here's the TL;DR version:

- Achievements have a base score of 10. This is done because Steam hasn't got a score system like Xbox and PlayStation do. This score is the True Steam Achievement Score - or TSA for short.
- Said score is modified based on how rare it is (determined by the number of players who've earned it), which sets its actual TSA value. (The exact formula is detailed in the link above).
- Rare achievements are thus given a higher point value based on the ratio of players who've earned it vs. players who haven't.

- The value can vary a bit because achievements become more common as more players earn it. For example, I was at 51,208 before writing this post, but as I write this I'm at 51,308. It's just there to give you a rough idea of how many achievements a player has earned - just like the gamerscore system does.

In case you (or anyone else) still wanted to know, there's a rough idea + an official post fully explaining it.