Interpretacja przedziału ufności


16

Uwaga: z góry przepraszam, jeśli jest to duplikat, nie znalazłem podobnego q w moim wyszukiwaniu

Powiedzmy, że mamy prawdziwy parametr p. Przedział ufności C (X) to RV, który zawiera p, powiedzmy 95% czasu. Załóżmy teraz, że obserwujemy X i obliczamy C (X). Częstą odpowiedzią wydaje się być to, że błędne jest interpretowanie tego jako mającego „95% szansy na zawarcie p”, ponieważ „zawiera albo nie zawiera p”

Powiedzmy jednak, że wybieram kartę z wierzchu potasowanej talii i zostawiam ją zakrytą. Intuicyjnie myślę o prawdopodobieństwie, że ta karta będzie Asa pik jako 1/52, mimo że w rzeczywistości „to jest albo nie jest to As pik”. Dlaczego nie mogę zastosować tego rozumowania do przykładu przedziału ufności?

Lub jeśli nie ma sensu mówić o „prawdopodobieństwie”, że karta jest asem pik, ponieważ „jest lub nie jest”, nadal stawiałbym szanse 51: 1, że nie jest to as pik. Czy istnieje inne słowo opisujące tę informację? Czym różni się ta koncepcja od „prawdopodobieństwa”?

edytuj: Być może by było bardziej jasne, na podstawie bayesowskiej interpretacji prawdopodobieństwa, jeśli powiedziano mi, że zmienna losowa zawiera p 95% czasu, biorąc pod uwagę realizację tej zmiennej losowej (i żadnych innych informacji, które można uzależnić) poprawne jest powiedzenie, że zmienna losowa ma 95% prawdopodobieństwo, że zawiera p?

edytuj: także, z częstokształtnej interpretacji prawdopodobieństwa, powiedzmy, że częstokładca zgadza się nie mówić czegoś takiego jak „istnieje 95% prawdopodobieństwa, że ​​przedział ufności zawiera p”. Czy nadal logiczne jest, aby częsty miał „pewność”, że przedział ufności zawiera p?

Niech alfa będzie poziomem istotności, a t = 100-alfa. K (t) to „pewność” częstego, że przedział ufności zawiera p. Ma to sens, że K (t) powinno wzrastać wt. Gdy t = 100%, częsty powinien mieć pewność (z definicji), że przedział ufności zawiera p, abyśmy mogli znormalizować K (1) = 1. Podobnie, K (0) = 0. Przypuszczalnie K (0,95) jest gdzieś pomiędzy 0 i 1, a K (0,999999) jest większy. W jaki sposób częsty uważa, że ​​K różni się od P (rozkład prawdopodobieństwa)?


1
Indeed, consider a coin flip where the coin rolls under a table, out of view and we consider the event that the coin landed on heads. At first glance this seems to be very similar to the CI issue - clearly either the event happened or it didn't. Yet in the coin flip case many (perhaps even most) frequentists seem perfectly happy to assign a notional probability, (say p) to the unobserved coin having ended up on heads, while backing away from saying the same thing about the random interval containing the parameter. To me there seems to be an inconsistency.
Glen_b -Reinstate Monica

@Glen_b Częstotliwość w scenariuszu z nieobserwowanym upuszczeniem monety stosuje przeciwne uzasadnienie, aby powiedzieć, że nie faktyczna wartość monety jest „losowa” (choć nie jest obserwowana), ale że możemy uogólnić każdy zaobserwowany wynik na inne potencjalne wyniki w tym upuszczonym monety i oblicz prawdopodobieństwa. Jeśli chodzi o prawdopodobieństwo rzeczywistej wartości nominalnej monety, jest ona albo nie jest głową, nie ma prawdopodobieństwa. jest zapisywany do alternatywnego budowy tego ustawienia. p
AdamO,

@Glen_b: Zgadzam się, patrz moje pytanie tutaj: stats.stackexchange.com/questions/233588/…
vonjd

@vonjd to what extent is your question there not simply a duplicate of the first paragraph after the opening "Note:" here?
Glen_b -Reinstate Monica

@Glen_b: To be honest I wasn't aware of this question when I posted mine and they certainly overlap. Yet I think they are not duplicates because mine is more generally concerned with using probabilities for hidden outcomes (which would have consequences for confidence intervals) whereas this one is purely aiming at confidence intervals. But if you think that mine is a duplicate feel free to close it.
vonjd

Odpowiedzi:


8

I think lots of conventional accounts of this matter are not clear.

Lets say you take a sample of size 100 and get a 95% confidence interval for p.

Then you take another sample of 100, independent of the first, and get another 95% confidence interval for p.

What changes is the confidence interval; what does not change is p. That means that in frequentist methods, one says the confidence interval is "random" but p is "fixed" or "constant", i.e. not random. In frequentist methods, such as the method of confidence intervals, one assigns probabilities only to things that are random.

Pr(L<p<U)=0.95(L,U)L= "lower" and U= "upper".) Take a new sample and L and U change but p does not.

Let's say in a particular instance you have L=40.53 and U=43.61. In frequentist methods one would not assign a probability to the statement 40.53<p<43.61, other than a probability of 0 or 1, becuase nothing here is random: 40.53 is not random, p is not random (since it won't change if we take a new sample), and 43.61 is not random.

In practice, people do behave as if they're 95% sure that p is between 40.53 and 43.61. And as a practical matter, that may often make sense. But sometimes it doesn't. One such case is if numbers as large as 40 or more are known in advance to be improbable, or if they are known to be highly probable. If one can assign some prior probability distribution to p, one uses Bayes theorem to get a credible interval, which may differ from the confidence interval because of prior knowledge of which ranges of values of p are probable or improbable. It can also actually happen that the data themselves --- the things that change if a new sample is taken, can tell you that p is unlikely to be, or even certain not to be, as big as 40. That can happen even in cases in which the pair (L,U) is a sufficient statistic for p. That phenomenon can be dealt with in some instances by Fisher's method of conditioning on an ancillary statistic. An example of this last phenomenon is when the sample consists of just two independent observations that are uniformly distributed in the interval θ±1/2. Then the interval from the smaller of the two observations to the larger is a 50% confidence interval. But if the distance between them is 0.001, it would be absurd to be anywhere near 50% sure that θ is between them, and if the distance is 0.999, one would reasonably be almost 100% sure θ is between them. The distance between them would be the ancillary statistic on which one would condition.


Thanks Michael that makes a lot of sense. Let's suppose in your example that we have a particular (L,U) but the values are not known to us. All we know is that it's the realization of a the 95% confidence interval random variable. Without any prior on the parameter or any other information, would it be fair to lay 19:1 odds that (L,U) contains the parameter? If a frequentist is willing to do this, but not call his "willingness to lay 19:1 odds that it contains the parameter" a "probability", what would we call it?
applicative_x

Yes, that probability is 0.95. Certainly within frequentist methods one can say that in a state of ignorance of (L,U) the probability is 0.95 that that interval contains p. But when one has particular values, which are not random, the frequentist will not assign a probabliity other than 0 or 1 to the statement, since the known values of L and U are not random.
Michael Hardy

4

The textbook definition of a 100×(1α)% confidence interval is:

An interval which, under many independent replications of the study under ideal conditions, captures the replicated effect measurement 100×(1α)% of the time.

Probability, to frequentists, comes from the notion of "rewinding time and space" to replicate findings, as if an infinite number of copies of the world were created to assess a scientific finding again and again and again. So a probability is a frequency exactly. For scientists, this is a very convenient way to discuss findings, since the first principle of science is that studies must be replicable.

In your card example, the confusion for Bayesians and Frequentists is that the frequentist does not assign a probability to the face value of the particular card you have flipped from the deck whereas a Bayesian would. The frequentist would assign the probability to a card, flipped from the top of randomly shuffled deck. A Bayesian is not concerned with replicating the study, once the card is flipped, you now have 100% belief about what the card is and 0% belief that it could take any other value. For Bayesians, probability is a measure of belief.

Note that Bayesians do not have confidence intervals for this reason, they summarize uncertainty with credibility intervals.


Thanks for the response. In the card example, wouldn't both the bayesian and frequentist agree that 51:1 is fair odds that the card is the ace of spades? Similarly, for the realization of a 95% confidence interval (and no other information), wouldn't both lay 19:1 odds that it contains the true parameter? In that sense, could a bayesian interpret the 95% confidence interval as having a 95% chance of containing the true parameter?
applicative_x

@applicative_x What about a pinochle deck? You are considering the use of prior information. The frequentist may only hypothesize that the probability is p=1/52 and only use the card's face valueto inform whether this experiment was consistent or inconsistent with that hypothesis. The validity of any type of interval estimate (credibility or confidence) depends on unverifiable assumptions. There is no such thing as a true parameter, this is a dangerous way of thinking about science. Bayesians do not play with confidence intervals per the earlier definition. Reread the answer.
AdamO

Thanks Adam, I think I am still confused. Let's suppose I know (by looking at the cards) that a 52 card deck is standard. I shuffle the deck and pick out the top 10 cards without looking at them. Couldn't I define the "true parameter" in this case to be the number of red cards? Then regardless of bayesian vs frequentist there is a "true parameter." If I am allowed to pick 7 cards at random I could also imagine constructing a confidence interval for the #of red cards out of my 10.
applicative_x

1
A Bayesian doesn't have to believe there is no such thing as a true value of a parameter. Bayesianism just means assigning probabilities to statements that are uncertain, regardless of whether they are random. A Bayesian can assign probability 1/2 to the statement that there was life on Mars a billion years ago. A frequentist cannot do that, since one cannot say that that happened in half of all cases. Nothing in that says a Bayesian cannot believe there is a true answer to the question whether there was such life on Mars. See also my posted answer to your question.
Michael Hardy

1
@AdamO : I find your comments cryptic. 'of what utility is the notion of "truth"' is a change of subject. "We think of truth as immutable." So "we" means you and who else, and what is the relevance of what they think? "No scientist would ever go about collecting data for the sake of verifying something that's already known." That seems like another change of subject. Then there follow some comments on frequentists and Bayesians. I don't feel like guessing what you're trying to say.
Michael Hardy
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