(15.05.2020)

As more tests are conducted, the number of new cases of covid-19 inevitably goes up. Then there is the issue of geographical variation. Though the number of new cases is falling in New York City, an early centre of the outbreak, the trend in the rest of the country is less certain. Finally there is the problem of data quality. The number of reported new cases tends to be higher at weekends and on Mondays, and lower on Thursdays and Fridays, for example. Adjusting for such seasonality can be tricky; a peak or valley on any given day can be accentuated by a number of other factors, not least variation in tests performed.