The key word is *exactly*. If I flip a fair coin, I expect about half of my flips to be heads. If I flip it twice, exactly one head is quite likely. If I flip it twenty times, exactly ten heads is not that unusual, but still a little lucky maybe. If I flip that coin one million times and get exactly 500,000 heads, well, that's quite unlikely indeed (the probability is about 0.08%, as it happens). Getting *exactly* the expected number of a random binary event gets less and less likely the larger the population gets.
If you think of being a smoker or not as a random binary event, like the coin flip, then the expected fraction of smokers in the married population might be equal to the (actual) fraction of smokers in the general population, but the chance of *actually having the exact number* of married smokers that would make those fractions equal is very small when discussing populations in the millions.
r~~
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2021-06-04T16:50:39Z (almost 3 years ago)