Day in and day out, the Director General of Health broadcasts the number of daily positive cases. Occasionally, the press release is sprinkled with additional details on deaths, ICU utilisation and the Rt (R at a particular point in time, t). R is the effective rate of reproduction, or effective reproductive number — it is the expected number of new infections caused by an infected individual in a population, where some individuals may no longer be susceptible.

Other commentators then jump in to provide more colour. Of late, the flavour of the day is the “daily test positivity rate” — the number of positive cases divided by the number of tests for the day (although we can safely assume the tests were not all taken on the same day). These commentators will go on to postulate how many more positive cases there will be if we were to increase the number of tests. We see viral news and videos, with nice charts and well-spoken doctors, saying how the number of positive cases will double if we double the number of tests. You sample a part of the population because you cannot test the entire population daily. The reason for sampling is to gauge its representation of the population, to facilitate making informed decisions, assuming the sample is unbiased (or corrected for intentional biasness). That is all.

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