It’s going down if you count from 2000 but up if you count from 2015. I’m not sure whether it makes more sense to look at the trend over the last 5 or 20 years

It’s going down if you count from 2000 but up if you count from 2015. I’m not sure whether it makes more sense to look at the trend over the last 5 or 20 years but I think you must make an argument that one should pick the span that supports your thesis, rather than the one that contradicts it.