Yancy Street Gang wrote: As for OPS, it's not a percentage, it's the sum of two percentages. I never understood the rationale behind that, other than that's it's an easy way to look at two percentages at once. The .900, though, doesn't mean 90 per cent of anything. That's my problem with OPS. It doesn't mean anything; you're adding two fractions with different denominators and treating them as equal, and they're not. That's why I always liked the other, more mathematically nice numbers, like TA and ISO.