Bad graph hunting

Much can go wrong when making data visualisations.

Anthony B. Masters
3 min readApr 24, 2022

Creating graphs is a key way to communicate statistics. There are many steps to this process, and graphs have a grammar.

Studying bad examples is often a good way to learn. Through failure, gain insights into the means of success.

Google cards

The Google card for reported Covid-19 surveillance deaths is inaccurate. The rolling average calculation is incorrect.

(Image: Google)

Their graph treats non-reporting days as if those values were missing. Reported deaths on days the agency does not update are, by definition, zero. For 21st April 2022, the seven-day rolling average should be about 239. Surveillance deaths by date of report were, in those last three days: 682, 508, and 482.

Since the agency did not report on the other four days, those other figures are zero. To get the seven-day rolling average, we sum and divide by seven.

(Image: UK Health Security Agency)

The Google graph performs a different calculation, averaging non-zero days. If their figures matched the UKHSA dashboard, the calculation would give about 557. These differences appear to arise…

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Anthony B. Masters

This blog looks at the use of statistics in Britain and beyond. It is written by RSS Statistical Ambassador and Chartered Statistician @anthonybmasters.