*Disappointed black guy*
"Black kid
stabbing white kid
over minor dispute"
"White woman
calling Black
kid N word
over minor dispute"
Jeremy Kauffman 🦔🌲🌕 @jeremykauffman: "The ban on gay blood donations was completely sensible and grounded in statistics. A gay man's blood _that tests negative for HIV_ still has roughly the same odds of having HIV as a straight man's blood that wasn't tested at all. It sounds crazy, but it's true."
"We can answer this with Bayes rule
P(HIV | straight, negative test) = P(negative test | HIV) P(HIV | straight) / P(negative test | straight)
= 0.3% x 0.2% / P(negative test | straight)
The probability of a negative test is P(negative test | HIV) P(HIV|straight) + P(negative test| no HIV) P(no HIV|straight), which is 0.3% x 0.2% + 98.5% x 99.8% = 98.3%. Plugging this in yields:
P(HIV | straight, negative test) = 6.1e-6
Now lets repeat this calculation for a gay person.
P(HIV | gay, negative test) = P(negative test | HIV) P(HIV | gay) / P(negative test|gay)
= 0.3% x 36% / (0.3% x 36% + 98.5% x 64%)
= 0.17%
That's a lot higher.
Edit 2016/06/25: Using Tom's better estimate the actual number is 0.09%. So getting 2 pints of blood from 2 independent gay men with negative HIV results is roughly as dangerous as getting 1 pint of untested blood from a straight man.
Base rates matter
Suppose a non-gay person donates blood. No HIV test is run on this blood. The risk of this blood having HIV is 0.2%. Now suppose a gay person donates blood, and an HIV test turns up negative. The risk of this blood having HIV is only a little bit less than 0.2%. In short, allowing gay people to donate blood is nearly as dangerous as letting straight people donate blood and then not testing it! I guess this twitter meme is wrong. The world does make sense. It turns out the Red Cross isn't anti-gay - they just did the math and decided to protect people who receive donated blood."
