Speedy Covid tests are very useful, but not conclusive | David Spiegelhalter and Anthony Masters

Behind the numbers: why the devices being used in England's schools need another test to confirm positive cases

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Last Monday, schools in England returned, with secondary school students having lateral flow" tests (LFTs). These device kits operate like pregnancy tests - a liquid sample flows along a pad. Instead of expecting a baby, positive tests mean students and families are expected to isolate.

All tests are imperfect, but the statistics of diagnostic tests are notoriously confusing.