alt Hacker NewsIt's contextual depending on which end of the probability is the desired outcome.
SPF is like this. SPF 30 allows 1/30th (3%) of the UV through it, blocking 29/30ths (97%). SPF 50 (2%) allows 1/50th, blocking 49/50ths (98%). Using the denominator, in this case, expressed the efficacy much more intuitively.
Comparing SPF 30 vs 50 better expresses the increase in efficacy than 97% vs 98% does.
One could also express it as the amount that passes the filter but it is Sun _Protection_ Factor not Sun _Transparency_ Factor.
On the other hand, it's completely unintuitive that "Sun Protection Factor 30 = 1/30th of UV light passes through it", and I had no idea there was any correlation before this comment.
The only intuitive bit about that system is "bigger number does more." I feel like I would have more readily understood it if it just said "blocks 97% (or 98%) of UV light" instead of numbers I assumed were somewhat arbitrary.