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> P=0.05 means that there is a 5% chance that the null is true No. It simply does not mean that. It would be very convenient if it did, but it does not and can
by sfink 9y ago
> P=0.05 means that there is a 5% chance that the null is true
No. It simply does not mean that. It would be very convenient if it did, but it does not and can not.
p=0.05 means that if the null hypothesis were true and you ran your experiment, then you would only have a 5% chance of getting the results that you did.
The distinction is not at all obvious, but you'll need to understand it before you can make sense of statistics.
The p-value is not the number you want; what you want is the probability that the null is false, but you can't have that unless you know the actual probability distribution. Which generally isn't something you could possibly know or figure out.
(You can make educated guesses about that distribution, by making a guess and then updating it based on observed results. That's Bayesian statistics, which is great because it does give you a full probability distribution and all you have to feed into it is... uh... a probability distribution.)
- deleted 9y ago[deleted]
- lisper 9y agoYes, you're right. I was making the tacit assumption that the null is true most of the time. On this assumption, the two statements are more or less equivalent. I believe this assumption is correct. If it weren't, making real scientific discoveries would be a lot easier. But it is an assumption.
- 3JPLW 9y agoYou're falling prey to the very common "p-value fallacy." P-values talk about the likelihood of the data, not the likelihood of the hypothesis. The two are unfortunately not linked.
- lisper 9y agoYes, I am aware of this. P-values tell you the probability that what looks like a positive result is not in fact a positive result but merely a fluke, i.e. the p-value is (a lower bound on) the probability of a false positive. So?