Tuesday, September 15, 2020

SAGE versus reality

Something I've been meaning to do for a while is look at how well the SAGE estimates of the growth rate of the epidemic have matched up to reality over the long term. For the last 3 months now, SAGE have published a weekly estimate not only of R but also the daily growth rate, which is actually a more directly interpretable number (as well as being provided to a higher degree of precision). What I have done is taken their estimate of daily growth rate and integrated it over time. And plotted this against the number of cases actually reported.

Here we are:

The solid blue line is the central estimate from SAGE, with the dashed lines calculated using the ends of the range they published each week. Red is the weekly mean number of cases over this time period, with this line scaled to start at the same place in week 1 (ending on Friday 19 June). Latest SAGE estimate in this plot is from Friday 11 Sept.

Agreement was very good for the first few weeks, with case numbers going down at the rate described by SAGE of about 3% per day. But then the case numbers started to drift up in July...and SAGE continued to say the epidemic was getting smaller. Over the last few weeks the discrepancy has grown sharply. Note that the dashed lines assume the extreme edge of the range presented by SAGE, week after week - so this would require a consistent bias in their methodology, rather than just a bit of random uncertainty.

Honesty compels me to point out that the comparison here is not completely fair, as the number of cases may not be a consistent estimate of the size of the outbreak. Some of the rise in cases may be due to increased testing. However the discrepancy between case numbers and the mean SAGE estimate is now a factor of 10 compared to the starting point of this analysis. That's not due to better testing alone!


Graeme said...
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Graeme said...
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CJB said...

Hi James, thanks for all your very interesting tweets. I wonder what you thought of the KCL app? Their estimated number of daily new cases has been increasing more slowly for a couple of weeks now (10-15% increase over that time (today about 22k)). That would suggest we may be nearing a plateau or peak, but that we might see 150-200 deaths in a few days' time and that this will continue for several weeks? hope you and Jules are both well!