Thread by @deeksj

Great that @ab4scambs shows some MPs understand the laws of probability (beginning to doubt that there was one) but this justification why we don't need to PCR kids who have positive LFT sadly starts with a fatal flaw

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conservativehome.com/platform/2021/…

This is the second time I have tweeted this as one decimal place went for a wander in my first set of tweets. Nothing else changes. Thanks to @d_spiegel for spotting it.

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It presumes that the prevalence to use for Covid infection in these calculations is that in the general population - 0.5% or 1 in 200. And then shows that 30% of those who are LFT+ and then PCR - will still have Covid infection. Can you spot the error?

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Only the asymptomatic kids get tested with LFT - if you have symptoms you go straight for a PCR test and never get to school. And @MattHancock tells us most days that one third of infections are asymptomatic (not sure how he knows that, but he is very sure).

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So we should start by dividing 0.5% by 3 - so 0.17%. Then the probability of Covid infection in somebody testing LFT+ve then PCR-ve will be 13% not 30%. This means that one in eight of those LFT+ve then PCR-ve will have infection, not 1 in 3 as @ab4scambs claims.

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If @DHSCgovuk is concerned that this is still too much of a risk, then get this group to have a second PCR test - the probability will drop below 1%. For every kids having an extra test we would stop around 1000 people having to isolate unnecesssarily for 10 days

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BUT ... neither of these sets of numbers match @DHSCgovuk data on tests in schools. Last weeks data: 311430 tests in secondary schools were reported on - @ab4scambs would predict 873 would be positive, my adjustment reduces it to 359 - but in reality only 175 were positive.

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To get my numbers to fit reality, either the test sensitivty has to dropped to 25%, or the prevalence of asymptomatic infection dropped to 0.05% - 1 in 2000 - far from @ab4scambs starting point of 1 in 200.

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