Permalink Submitted by Susan Mitchell on June 23, 2020

In order to make sense of growth rates we need halving times (the time in days taken for current daily numbers of new coronavirus cases to half) for negative growth rates and doubling times (the time taken in days for current daily numbers of new coronavirus cases to double) for positive growth rates.

This would make more sense to the general public than growth rate percentages and the general public could then better predict what might happen to daily numbers of new Covid-19 cases in the future. This would allow us all to hold government to account because we could tell whether the government's predictions are correct.

However, there is a problem. The government figures for growth rates are only given to the nearest percent. This is problematic because doubling and halving times increase markedly near a growth rate of zero (and where R=1). We need to have growth rates to a precision of at least a tenth of a percent. Otherwise at anywhere near a growth rate of zero or an R of 1 the figures given are useless. At whole numbers of percents we cannot calculate a halving time or doubling time of more than 68 days (2 months). The next stop is infinity.

If we are given growth rates to the nearest tenth of a percent we can calculate a halving or doubling time of up to 690 days (2 years).

The difference is extremely important because we not only need to know whether growth rates and R numbers are above or below zero and 1, respectively. We also need to know whether they are actually or effectively zero or 1. If this is the case then the daily numbers will stay the same for the foreseeable future.

This has huge ramifications. If the halving (or doubling) time is two years we won't notice much of a drop (or rise) in the next few months. Hospitals will still have the same number of coronavirus cases to deal with every day and other medical interventions and cancer testing will continue to stay on hold. If the halving (or doubling) time is even longer then, in the absence of good quality track and trace or a vaccine, the only thing that will save us is herd immunity.

## Decimal places, halving times, doubling times

In order to make sense of growth rates we need halving times (the time in days taken for current daily numbers of new coronavirus cases to half) for negative growth rates and doubling times (the time taken in days for current daily numbers of new coronavirus cases to double) for positive growth rates.

This would make more sense to the general public than growth rate percentages and the general public could then better predict what might happen to daily numbers of new Covid-19 cases in the future. This would allow us all to hold government to account because we could tell whether the government's predictions are correct.

However, there is a problem. The government figures for growth rates are only given to the nearest percent. This is problematic because doubling and halving times increase markedly near a growth rate of zero (and where R=1). We need to have growth rates to a precision of at least a tenth of a percent. Otherwise at anywhere near a growth rate of zero or an R of 1 the figures given are useless. At whole numbers of percents we cannot calculate a halving time or doubling time of more than 68 days (2 months). The next stop is infinity.

If we are given growth rates to the nearest tenth of a percent we can calculate a halving or doubling time of up to 690 days (2 years).

The difference is extremely important because we not only need to know whether growth rates and R numbers are above or below zero and 1, respectively. We also need to know whether they are actually or effectively zero or 1. If this is the case then the daily numbers will stay the same for the foreseeable future.

This has huge ramifications. If the halving (or doubling) time is two years we won't notice much of a drop (or rise) in the next few months. Hospitals will still have the same number of coronavirus cases to deal with every day and other medical interventions and cancer testing will continue to stay on hold. If the halving (or doubling) time is even longer then, in the absence of good quality track and trace or a vaccine, the only thing that will save us is herd immunity.