How do you decide which champ needs to get nerfed/buffed

I used to think it's about winrates (and a lot of people too)... but nope. Pickrate? Not sure about this one too... For example current PBE Riven buff. She has decent playrate and 50.2%~ winrate. Or take a look at... let's say... Cho'Gath. 48%~ w/r and really low pickrate (I chosed Cho cuz his toolkit isn't really toxic as other low w/r champs like Urgot, Yorick, Azir, Kalista). I'm not whining about Riven but statistic shows that she perform ok in ranked. So-o... Ahem. Why?