THREAD: The "positivity rate" on which the future of the lockdown was decided is a particularly awful data point that tells a misleading story about #COVID19 infections. Take the travelers placed under mandatory quarantine in Accra and Tamale, for example. 1/9

@benkoku

1030 travellers were placed under mandatory quarantine in Accra on their arrival at KIA on March 21-22. Same for 10 Guinean nationals in Tamale who entered Ghana on March 18. That's a total of 1040 PERSONS. That's different from the 2022 listed on the GHS page. Why? 2/9

The 2022 listed in Table 2 is the number of SAMPLES collected from those 1040 PERSONS under mandatory quarantine. The disparity is because some were tested multiple times. 3/9

The question we are interested in is: what % of PERSONS under mandatory quarantine are infected with #COVID19. The numerator is the number of positive persons, and the denominator is the total 1040 persons under quarantine. This is the PREVALENCE. 4/9

Prevalence is an important epidemiological variable for understanding the extent of spread of #COVID19 in a population. 115 of the 1040 travelers returned at least one positive result during their quarantine - an 11.06% prevalence. 5/9

But for some reason we're reporting and using a so-called POSITIVITY RATE. This statistic appears to use the number of positive PERSONS as the numerator, but the number of SAMPLES tested as the denominator. That returns a rate of 5.69% (vs. 11.06% prevalence). 6/9

That's problematic because some travelers gave MULTIPLE samples. The effect is to increase the denominator and water down the rate. Such a statistic makes zero sense. It isn't even a valid measure because the units of measure of the numerator and denominator are different. 7/9

Shockingly, we seem to be basing the policy decisions on the POSITIVITY RATE - a terrible statistic produced from even worse maths - and ignoring the PREVALENCE, which is nearly TWICE as high. 8/9

This is why public health folks are so baffled. The decision to lift the lockdown is at best premature from an epidemiological point of view. But the main data point being used to justify it is shockingly disingenuous and lacking in basic scientific validity. 9/9