5 ms·
> By "cubic fit" I infer that they mean some kind of low-pass signal filter. It's literally just ax^3 + bx^2 + cx + d, optimize a, b, c, and d to minimize some
by CrazyStat 6y ago
> By "cubic fit" I infer that they mean some kind of low-pass signal filter.
It's literally just ax^3 + bx^2 + cx + d, optimize a, b, c, and d to minimize some loss function (probably L2).
- gowld 6y agoAnd since cubics can't be nearly flat over two disjoint intervals separated by an extrenum, they added an unlabelled nonsensical bell curve continuation to the to the end where a cubic would predict the case rate plummeting to negative infinity
- em500 6y agoNot really, they just fitted the cubic to log-deaths and exponentiated them again.
- CrazyStat 6y agoCubic has to go to +infinity on either the right or left. Exponentiating doesn't help with that.
- em500 6y agoexp(-infinity) = 0
- CrazyStat 6y agoAnd exp(+infinity) = +infinity, so it has to go to +infinity on one side.
- em500 6y agoYes, on the right side, https://statmodeling.stat.columbia.edu/2020/05/16/what-a-difference-a-month-makes-polynomial-extrapolation-edition/ https://statmodeling.stat.columbia.edu/2020/05/16/what-a-dif... So in addition to log-exp to prevent projections going negative, they clipped the end date at 4 Aug to prevent it from going back up to +inf