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I'm just going to comment on the graph, not the tweets. The graph shows three predictions from IHME. One, the blue line, is a model from 3/27. It matches the d
by scythe 6y ago
I'm just going to comment on the graph, not the tweets. The graph shows three predictions from IHME.
One, the blue line, is a model from 3/27. It matches the data okay.
The second, the yellow line, is a model from 4/5. The agreement of this line is much worse, and the fact that the model did worse with more data is not promising.
The third, the teal line, is a model from 5/4. The data (black) ends at 5/4. So the agreement of the teal line with the black line is not a prediction at all.
The red line, a cubic fit, is totally irrelevant. By "cubic fit" I infer that they mean some kind of low-pass signal filter. Fitting a simple model like that to a complex time series without some motivation for why that model was chosen is the mathematical equivalent of treating tuberculosis with mercury.
My point is: it sure doesn't look like the models are doing a good job of predicting death rates. And that's just from the graph used to advertise them.
- 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