Suppose your middle-schooler came home with a math assignment.

She was told to measure everyone in the house and report on heights. But checking over the assignment, something seemed odd. Your 190cm partner wasn’t included.

“Oh, our teacher explained that most people are between 150 cm and 180cm. If we got a measurement outside that range, there was no guarantee that they were really that tall, so we should just ignore it.”

You’d worry about the state of numeracy among teachers, right?

Sure, the average person will be within that range. Some random person isn’t likely to be 190 cm tall. If the assignment had asked, “Is it likely that the next person you meet is 190 cm tall?”, the answer should be no – unless you happen to live with a very tall person.

But conditional on having been measured at 190cm, it’s more likely that you’re a tall person than that it’s just measurement error. If the ruler said three metres, you’d probably want to triple-check the figures. But 190cm isn’t that uncommon.

Why do I bring this up?

Back in May, I asked the Ministry of Health why they’ve been telling people to ignore positive RAT results after seven days of isolation. The Ministry replied this week. They told me that because seven days is beyond the average period of infectiousness, the current health guidelines do not require a negative test to leave isolation.

It wasn’t a cost-benefit assessment. It was a failure in basic numeracy similar to our imaginary teacher’s error – but far more consequential.

Work published in JAMA Internal Medicine in April suggested at least 60% of people with a positive RAT result six days in are still infectious.

The Ministry doesn’t seem to have checked any of that. They instead looked at the average period of infectiousness and decided positive results after that period could be ignored.

So people will be heading out, after isolation, on official advice, and sharing their Covid gift with others.

Yale Covid testing expert Dr Anne Wyllie had described the Ministry’s advice as ‘dangerous misinformation’.

I don’t know whether the Ministry’s answer reflects general incompetence, declining numeracy more broadly, or a dangerous mutant strain combining the two.

Let’s hope the revised NCEA numeracy guidelines filter their way up into the Ministry of Health before more dangerous variants arrive.