
Graeme M » June 16th, 2017, 9:01 pm wrote:
The author was observing that the chances of an event occurring change depending on whether the event has occurred or not. He wrote that he was issued a particular number plate and that the odds of this plate having the number/letter combination it did was one in 175,760,000 before he received the plate, but that "after the fact, the probability is one".
Graeme M » June 16th, 2017, 9:01 pm wrote:To my mind, the probability of an event can only be assigned prior to the event. Once it occurs, there is no probability at all, only certainty. Similarly, once the event occurs, its probability remains at whatever it was, in this case one in 175,760,000. That is, although it occurred, the probability of it occurring was what it was. Probability seems to me to be simply attempting to quantify the likelihood that an event might occur.
Braininvat » June 16th, 2017, 10:35 pm wrote:It's just semantic choices. Probability doesn't change for getting a plate, but our expectation of it changes after we get that plate. If there were a bin full of baby souls, randomly distributed to planets, then my chance of being born on Earth is .0000012, say. When I am born, the chance is 1.0 because I can't reasonably expect to be anywhere else. The chance for the other binbabes remains at .0000012. As NS said, it's about new information.
Graeme M » June 16th, 2017, 6:01 am wrote:To my mind, the probability of an event can only be assigned prior to the event. Once it occurs, there is no probability at all, only certainty.

someguy1 » June 17th, 2017, 4:33 am wrote:
Here is yet another real life example. I go to the airport. The authorities give me a full body cavity search. They examine the contents of my luggage. They take my laptop apart down to the screws and chips.
What is the probability that the flight has been made safer?
someguy1 » June 16th, 2017, 3:33 pm wrote:
The point being that probability is not anything inherent in the event itself. Rather, probability is a measure of our state of knowledge of the event.
Graeme M » June 17th, 2017, 8:05 am wrote:someguy1 » June 16th, 2017, 3:33 pm wrote:
The point being that probability is not anything inherent in the event itself. Rather, probability is a measure of our state of knowledge of the event.
Yes, you see that's exactly what confuses me. That "definition" is precisely what I'd have naively expected a quantified probability to mean. It is simply a measure of our uncertainty about a future event. But once an event occurs, there is no longer any uncertainty and hence no probability. To my mind we cannot talk about the "probability" of an event that has occurred. Our state of knowledge after an event is a different thing, so it seems to me!
I think this just means some kind of semantic thing on my part  I just don't see how we can talk of a "probability" for a past event, except in terms of what the probability was prior to its happening.
But you are saying that we can still talk of the probability of an event after the fact, it's just that now we know that the event has occurred and hence its probability is one. Fair enough.
Still doesn't sound reasonable to me though! :)
Braininvat » June 16th, 2017, 5:04 pm wrote:Someguy: becoming a Big Fan of your lucid style of explanation.
Graeme M » June 17th, 2017, 8:50 am wrote:Well yes, I think the article was simply discussing probability in a deterministic sense, rather than touching on quantum indeterminacy. The author was discussing statistical probability and "survivor bias" whilst reviewing a book called "Standard Deviations".
Graeme M » June 17th, 2017, 8:50 am wrote:In the book another example is given:
The odds of me having a particular playing hand in a game of poker is about three million to one, but the book's author states that "After I look at the cards, the probability of having these cards is 1, not 1 in 3 million".
Graeme M » June 17th, 2017, 8:50 am wrote:I disagree. The probability that I have that hand remains the same, the fact that I do have them is now in evidence. Probability must surely be a measure of ignorance, not knowledge.
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