Now, if you correctly ignore the pre 2010 weight 2nd and 3rd value as having P(W2|W1)=1 and P(W3|W1) = 1 you will get one result. But if you treat all the values as repeated independent measurements of the weight, you will find that before 2010 there was a LOT less variability in weight among patients.

Again, updating your posterior using the correct mathematical expression for the probability will result in the correct answer, but updating your posterior using a "default" expression for the probability that fails to take into account the method of data collection and data tabulation... results in the wrong answer because you "used some data multiple times". The real reason is "you didn't condition correctly on your knowledge of the data tabulation process"

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