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Dale
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Why not? Since that is the specific question of interest that is exactly what we should do.PeroK said:We're not estimating ##\lambda##,
Why not? Since that is the specific question of interest that is exactly what we should do.PeroK said:We're not estimating ##\lambda##,
Dale said:If you had previous studies that showed, for example, that couples who decided on a fixed number of children in advance had different ##\lambda## than other couples.
I agree. I certainly would assume equal priors, but in principle they could be unequal.PeterDonis said:I think this gets into complications that I said I didn't want to get into in this thread.
If you gave me some data that read ##XXXXXXY## and you asked me to estimate the probability of getting ##X## or ##Y##, then (if forced to give an answer) I would say ##6/7## for ##X##.Dale said:Why not? Since that is the specific question of interest that is exactly what we should do.
Yes. This is roughly the way that frequentist statistics would do it. I think the “official” process would be a maximum likelihood estimator, but that is probably close.PeroK said:If you gave me some data that read ##XXXXXXY## and you asked me to estimate the probability of getting ##X## or ##Y##, then (if forced to give an answer) I would say ##6/7## for ##X##.
Well, the calculation that he is making is not an estimate of ##\lambda##. I think that the frequentist estimate of ##\lambda## would be the same for both couples. What would differ is the p value.PeterDonis said:OTOH, if the frequentist claim @PeroK is making is right, then there ought to be some way of reflecting the difference in the Bayesian calculation as well. But I can't come up with one.
Dale said:This is roughly the way that frequentist statistics would do it.
Dale said:I think that the frequentist estimate of ##\lambda## would be the same for both couples. What would differ is the p value.
Dale said:Since the p value isn’t part of Bayesian statistics the fact that it distinguishes between the two couples may not have a Bayesian analog.
Dale said:the calculation that he is making is not an estimate of ##\lambda##.
Most treatments of this type of problem that I have seen would use a Beta distribution since it is a conjugate prior. So you would get ##\lambda \sim Beta(2,7)## for the posterior for both cases separately or ##\lambda \sim Beta(4,14)## if you were pooling the data for an overall estimate.PeterDonis said:In that case, a Bayesian would use a maximum entropy prior, which basically means that your posterior after the first set of data is whatever the distribution of that data set is.
Frequentist confidence intervals will be different between the two couples, and Bayesian credible intervals will be different from either of those. But as far as I know Bayesian credible intervals will be the same for both couples. That is precisely the advantage of Bayesian methods highlighted in the paper I cited earlier. This is, in fact, a fundamental difference between the methods.PeterDonis said:But the p-value affects our confidence level in the estimate, correct? So the confidence levels would be different for the two couples.
Well, the narrow question is clear and can be answered. I am not sure that the broad question is sufficiently well defined to be answerable.PeterDonis said:Again, "estimate ##\lambda##" might not be the right way to express what I was asking in the OP. I did not intend the OP to be interpreted narrowly, but broadly
Dale said:That is precisely the advantage of Bayesian methods highlighted in the paper I cited earlier.
I do not think that the fact that there are different p-values does or should mean that our posteriors should be different.PeterDonis said:So far I have only one difference that has been described: the p-values are different. Are there others? And what, if any, other implications does the difference in p-values have? Does it mean we should have different posterior beliefs about λλ\lambda?
Dale said:I do not think that the fact that there are different p-values does or should mean that our posteriors should be different.
(this is not really on topic for the thread, but you asked and it is a topic that I am somewhat passionate about, so ...)PeterDonis said:Why is it an advantage? Why are Bayesian credible intervals right and frequentist confidence intervals wrong?
PeterDonis said:Again, "estimate ##\lambda##" might not be the right way to express what I was asking in the OP. I did not intend the OP to be interpreted narrowly, but broadly.
Perhaps a better way to broadly express the OP question would be: there is obviously a difference between the two couples, namely, that they used different processes in their child-bearing process. Given that the two data sets they produced are the same, are there any other differences that arise from the difference in their processes, and if so, what are they? (We are assuming, as I have said, that there are no other differences between the couples themselves--in particular, we are assuming that ##\lambda## is the same for both.)
So far I have only one difference that has been described: the p-values are different. Are there others? And what, if any, other implications does the difference in p-values have? Does it mean we should have different posterior beliefs about ##\lambda##?
Dale said:p-values depend on your intentions
Yes, in fact it is the key issue. The only difference between the couples was their intentions. Frequentist methods are sensitive to the intentions of the experimenters as well as the analysts. Did you read the paper? It covers both.PeterDonis said:This might be an issue in general, but it is not in the particular scenario we are talking about here.
PeroK said:This probably only makes sense if we allow a second parameter - for example that some couples have a predisposition for children of the one sex. Otherwise, there no reason to doubt the general case.
PeroK said:What this data does question is the hypothesis that no couples have a predispostion to the one sex or other of their children.
PeroK said:all we can do is calculate how unlikely each of these families is under the hypothesis that in general ##\lambda = 0.5##. Nothing more
PeterDonis said:What is "the general case"? We are assuming for this discussion that there is no second parameter--p is the same for all couples.
If by "the general case" you mean ##p = 0.5## (or ##\lambda = 0.5## in @Dale's notation), then the actual evidence is that this is false; the global data seems to show a value of around ##0.525## to ##0.53##.
Yes, but does it question it to a different extent for couple #2 vs. couple #1? Does their different choice of process make a difference here?
Dale said:The only difference between the couples was their intentions.
PeterDonis said:However, I can see an argument here regarding the intentions of the couples: the gametes don't know at each conception what rule the parents were using to decide when to stop having children. So there is a straightforward argument from the biological facts of conception that the process the parents are using to decide when to stop having children should not affect the data.
PeroK said:my guess would be that the second family would be more likely to be one of the predisposed couples than the first
PeterDonis said:I have not done a Bayesian calculation with ##\lambda## treated as a function of the individual couple instead of an unknown single parameter, but it seems to me that such a calculation would still say that, since the data sets of both couples are the same, our posterior distribution over whatever parameters we are estimating will be the same. The key here is that the difference we have information about for the two couples--the way they choose to decide when to stop having children--has no relationship that I can see between any difference between them that would be expected to be relevant to a difference in ##\lambda## between the two couples.
In fact, even if we discount the subjective judgment I just expressed, and decide to test the hypothesis that "there is some difference between these two couples that affects ##\lambda##", the fact that the two data sets are identical is evidence against any such hypothesis!
PeroK said:certain things had to happen in order for a case #2 family to end up with seven children.
No, it is about the experimenters as well as the analysts. The couples are experimenters since they had an experiment with a stopping criterion and collected data. You really should read the paper.PeterDonis said:The intentions of the couples, not the researchers (us) who are evaluating the data. The p-value hacking issue is an issue about the intentions of the researchers.
PeroK said:You need to calculate what is implied by the assumptions in the problem and what is being compared to what.
Dale said:The couples are experimenters since they had an experiment with a stopping criterion and collected data.
Interestingly, there is an approach called hierarchical Bayesian modeling which does exactly that.PeroK said:Unless we allow the second parameter, all we are doing is picking up unlikely events. We can calculate the probability of these events, but unless we allow the second parameter, that is all we can say.
...
In summary, to make this a meaningful problem I think you have to add another parameter.
Dale said:In this model each poll is considered to have some underlying probability of a win (analogous to a couple's probability of having a boy) which is considered a "hyperparameter", then the respondents to the poll are binomial draws from the prior (analogous to each child being a draw from the couple's probability). The observed data then informs us both about the probability for each couple as well as the distribution of probabilities for the population.
Yes, you could do it that way. The details vary a little if you want to consider only these two stopping criteria or if you want to consider them as elements of a whole population of stopping criteria. The hierarchical model is more appropriate for the second case. Essentially this is the difference between a fixed effect and a random effect model.PeterDonis said:this methodology could provide a way of investigating questions like ...
YesPeterDonis said:the evidence described in the OP ... would be evidence against any hypothesis that the hyperparameter varied from group to group
PeterDonis said:One way of rephrasing the question is whether and under what circumstances changing the stopping rule makes a difference. In particular, in the case under discussion we have two identical data sets that were collected under different stopping rules; the question is whether the different stopping rules should affect how we estimate the probability of having a boy given the data.
I evidently misread the original post. Given this structure I opted to view it as a baby markov chain (pun intended?), and use renewal rewards.PeterDonis said:This is not the correct stopping rule for couple #2. The correct stopping rule is "when there is at least one child of each gender". It just so happens that they had a boy first, so they went on until they had a girl. But if they had had a girl first, they would have gone on until they had a boy.
I can try... it's an enourmously complex and broad question in terms of math, and then more so when trying to map these approximations to the real world. A classical formulation for martingales and random walks is in terms of gambling. The idea behind martingales is with finite dimensions a fair game stays fair, and a skewed game stays skewed, no matter what 'strategy' the better has in terms of bet sizing. With infinite dimensions all kinds of things can happen and a lot of care is needed -- you can even have a formally fair game with finite first moments but if you don't have variance (convergence in L2/ access to Central Limit Theorem) then extremely strange things can happen -- Feller vol 1 has a nice example of this (chapter 10, problem 15 in the 3rd edition).PeterDonis said:Can you give examples of each of the two possibilities you describe? I.e, can you give an example of a question, arising from the scenario described in the OP, for which stopping rules don't matter? And can you give an example of a question for which they matter a lot?
So, the easiest way to do this analysis is using conjugate priors. As specified by @PeterDonis we assume that both couples have the same ##\lambda##. Now, in Bayesian statistics you always start with a prior. A conjugate prior is a type of prior that will have the same distribution as the posterior. In this case the conjugate prior is the Beta distribution. If these were the first two couples that we had ever studied then we would start with an ignorant prior, like so:PeroK said:What does a Bayesian analysis give numerically for the data in post #1?
Dale said:we assume that both couples have the same ##\lambda##.
Dale said:When couples have a unusual ratio we automatically suspect random chance may be skewing the results a bit, but do admit that there is some possibility that there is something different with this couple so that the results are not totally random.