what does 2% failure rate per year even mean when it’s presented independent of a number of uses per year
This is a good observation. You can look up what the average number of uses per year is. If I remember right, I’ve seen some condom efficacy studies include that information.
I feel like I’m missing something basic here that would let me see why it’s a useful piece of information on its own.
You’re not missing anything basic, you’re correctly perceiving ambiguity where ambiguity does exist. Even when information is really important, I’ve found that it’s often been omitted simply because products are marketed to the average person, not to nerdy people like me, and most people don’t want to think as much as I do. For this reason, I’ve found it’s very important to be careful not to assume that the world is doing sensible things or giving me all the information. They’re not just leaving information out, they’re also not being held accountable by a world full of people who think as much as I do. Therefore, they can get away with slapping various nonsense marketing claims and out-of-context data on their boxes without people questioning them.
Here’s how 2% per incident is different:
Let’s say, hypothetically speaking, that the average number of uses per year is 100.
A 2% per incident risk will add up to a yearly 50% risk for the average user.*
A 2% per year risk already included 100 uses, so it is still 2% per year.
A 2% per year risk would add up to a 70% chance over the 35 or so years women are fertile and active and a 2% per incident risk would add up to a much, much higher risk, likely resulting in multiple pregnancies.*
* This is only if pure math reflects reality, which it probably doesn’t because there are other factors here like people forgetting important parts of the instructions over time, people getting better at using them over time, or people becoming sloppy about applying them because they’re tired of them or have developed a sense of over-confidence.