Why the doubling time is only meaningful

Whether Doctors, politicians, or entrepreneurs – in the fight against the Corona-crisis are all dependent on the most meaningful Numbers and Trends. The current data situation – for example, to the number of actually Infected, or for the mortality rate is from the perspective of experts such as the medical statistician Gerd Antes is still much too thin. You need reliable Numbers about as a basis for planning, because otherwise it is not foreseeable to be, “how big is the Tsunami that was coming to us,” says the former head of the German Cochrane centre at the University hospital Freiburg. It is about to advance to say how very the intensive care beds will be utilized and how high is the demand for ventilators will be.

Currently, there are for the Tests on a Sars-CoV-2 is still a very different practice, even with similar symptoms, origin from high-risk areas or contact with Infected is not like the decision for or against a Test unit, as Antes says. To find out how many people in Germany are infected, it would take a large-scale sample of the Population.

The issue of underreporting

Only with test, independent studies, let calculate, how many Infected there are in Germany, actually. “You must now be carried out very quickly and with high quality, even if you have to put millions on the table,” says the scientist. The Berlin-based virologist Christian Drosten wrote about this a few days ago on Twitter: “A possible dark point can be determined only by means of geographically broad-based investigations.”

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Despite the thin data, and these must be drawn for the decision-makers but now Trends. This happened so far with the so-called doubling time. You should take statements, how many days it is until the number of demonstrably Infected has doubled. The calculation seems simple, the meaning of the number is simple – the higher, the better.

Doubling time due to the meaningful

However, mathematically, there are problems: “The doubling time is calculated under the assumption that the number of known infections is growing exponentially,” says Moritz Neumann, an expert in Applied Analysis at the University of Bielefeld. However, the number of registered Infected is no longer growing exponentially. This is also due to the existing limitations in public life. Thus, the number loses its significance: “One can conclude, therefore, by no means on the future, as it is the sense of the doubling time actually,” says Neumann.

Reproductive rate as an Alternative

An Alternative to the doubling time is the so-called reproduction. It tells you how many people infects a of Infected, on average. The lower the value, the better. The reproduction rate is about 1, to put an Infected person in the middle more than other people – so the number of daily new infections increases. The Rate is less than 1, infects a of Infected in an average of less than other people – and the epidemic is running out gradually.

The calculation of the rate of reproduction is complex. Scientists can choose different models and have to estimate parameters. “With different methods come to different results,” says Neumann. The mathematician is recommended, therefore, for the communication about the development of the Corona the case of numbers, very simple, the number of daily new infections. The value says how many people were daily re-registered as Infected. The number of drops over several days, can be seen as a positive Trend to be understood.

Variations of individual days or the weekend to compensate for, is calculated in the rule, the mean value of several past days. But no matter what number you choose, one thing must be clear: The forecasts can be only as good as the Numbers from which they were calculated.

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