[introspectif]

"after rolling out our optimizations, we saw our error rates (ie. the proportion of slow or timed out queries) drop by up to 400%"

There is some good shared knowledge in the post (unlike this comment, to be fair), but what does drop by 400% mean?

If a rate drops by 100% it becomes zero. I get that.

If it increases by 400%, the outcome is slightly ambiguous (do we add 400% for 500% total or do we multiply up to 400% of the original value).

But a rate decreasing by 400% - am I the only person who finds that (not uncommon) expression hard to conceptualize?

[erichocean]

I understood it to mean that it had 1/4 the error rates they were previously seeing.

[dllthomas]

Huh, that's not even one I considered. I guessed 1/5, so the change was 4x the final value. Either way it's wrong - "dropped by X%" should be calculated as (original - final) / original.

[apurvamehta]

This is exactly right :)

[caf]

Then it dropped by 75%.

[apurvamehta]

Yes. It was a blunder. The post has been updated to reflect this.

[AsymetricCom]

Obviously, he gets -300% errors now.

[ihsw]

That's pretty good.