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"That's not even remotely what was done in the article by my reading." That's what it was described as in the grandparent: "This was a study based on "a Finan

• tgb 11y ago
  > 'mainland Chinese' are a reference class much larger than 'Chinese buyers of Vancouver properties', as they are buying globally. And 9/77 of mainland Chinese are buying properties in Vancouver, so you multiply that by total number of people in mainland China (that are high net worth and affluent) and then multiply that by the average property value bought by them. That gets (a rough approximation of) the property value bought by Chinese mainlanders in Vancouver, now divide that by the total property market in Vancouver to get the percent of Vancouver being bought by Chinese mainlanders. I don't know where you're getting anything else than this. The top level parent only described this as "some more mathematical shenanigans" yet you assume that therefore it has to be this nonesense of swapping P(A|B) with P(B|A). The article makes it clear that that's not the case, so I don't know who you're arguing with. > Which is still wrong, since there are lots of Somalis not in Minnesota. Yes, 67% of them in this hypothetical, as accounted for in the first factor. This process isn't inherently wrong, the debate is whether the small sample size makes the uncertainty too large to be useful.
  • aab0 11y ago
    > And 9/77 of mainland Chinese are buying properties in Vancouver, so you multiply that by total number of people in mainland China (that are high net worth and affluent) and then multiply that by the average property value bought by them. That gets (a rough approximation of) the property value bought by Chinese mainlanders in Vancouver, now divide that by the total property market in Vancouver to get the percent of Vancouver being bought by Chinese mainlanders. And now you've done the correction for the base rates which I discussed in my original: once you know the relevant populations and fractions, then yes, you can get out the right fraction. But it's highly unlikely the correct version will just happen to equal the original fraction in the survey, and no correction was included in the summary. Let me quote again what I am criticizing: "This was a study based on "a Financial Times multiple choice survey of 77 high net worth and affluent mainland Chinese individuals, 'admittedly not a statistically significant sample size,'” According to this survey, 9 individuals out of these 77 Chinese bought property in Vancouver." Note there is nothing about 'total number of people in mainland China'. Nothing about 'average property value'. Nothing about 'total property market in Vancouver'. I don't know where you're getting all this. The procedure as described is just plain wrong, nothing to do with sample sizes or Bayesian methods. > This process isn't inherently wrong, the debate is whether the small sample size makes the uncertainty too large to be useful. That's what the comment thought they were defending, but they were wrong. They misunderstood the problem entirely. It was interpreting the survey meaning totally incorrectly by inverting the conditional, the problem was not precision/sampling error.
    • tgb 11y ago
      There's nothing the least bit mathematically fallacious about the line you quote: it's just stating the results of a survey. You are critiquing something that has to occur after that point. The very next line from the source you're quoting: > After some more mathematical shenanigans, this is extrapolated to imply that Chinese mainlanders buy 33% of the Vancouver real estate market. I.e. "there's more that I'm not talking about because I already think the first step is so inaccurate that it doesn't matter what's next". Which seems more likely? (A) that the above snarkily referred to what was done in the article, or (B) that the above referred to a mathematical fallacy that is otherwise entirely absent from anything anyone here is talking about until you showed up. Even if the top poster were saying what you suggest, how were they supposed to get 33% out of 9/77 = 11%? Your suggested error would give 11% instead.