mm-3919 === Subject: Re: Formula to combine physical measurements to one? I thought one could incooperate this by correct weighting with the errors of each partial mean. What if I have different measurement methods, so that I get error bounds for each, but I can't just take the normal mean of these measurements without proper weighting (smaller error measurements have higher weight). === Subject: Re: Formula to combine physical measurements to one? I misunderstood your first post. If you are saying that from the first set of measurements you have an estimate x1 and an estimate s1 of its standard deviation (rather than the sample standard deviation of the measurements) and similarly for the second set, then a reasonable way to combine the estimates is to use x and s (the estimated sd of x) where x = a*x1 + (1-a)*x2 where a = s2*s2/(s1*s1 + s2*s2) s = s1*s2/sqrt( s1*s1 + s2*s2). You can derive this, for example, by assuming that x1 and x2 are independent, and by looking for the estimate of the form a*x1 + (1-a)*x2 (and so has the same mean as x1 and x2) which has minimum variance. === Subject: Re: Formula to combine physical measurements to one? Thats sounds reasonable to me, if even though I have experience in maths but not statistics to justify the last argument. Meanwhile I was thinking of something and now I wonder why the following gave me a different formula: Suppose I have a Poisson distribution. I perform a measurement N times and get a success rate of C counts. Now my scaled result is x=C/N (rate of positive events) s(x)=sqrt(C)/N (that's what I'm using for the error. correct?) If I had split one such experiment into two (say having a coffee break), then I had two estimates x1=C1/N1, x2=C2/N2 s1=sqrt(C1)/N1, s2=sqrt(C2)/N2 which should combine logically to x=(C1+C2)/(N1+N2) s=sqrt(C1+C2)/(N1+N2) if I had done the experiment in one go. This result is only obtained with the formula: x=(x1^2/s1^2+x2^2/s2^2) / (x1/s1^2+x2/s2^2) s=sqrt(x1^2/s1^2+x2^2/s2^2) / (x1/s1^2+x2/s2^2) which is different than above. Where's the mistake? === Subject: Re: Formula to combine physical measurements to one? Well if you observed k1 .. kN and were assuming a Poisson distribution, Sum{ k[i]}/N is the maximum likelihood estimator for the parameter of the distribution. The rule I mentioned before is how to get a minimum variance combination of two estimators. There's no particular reason why this should be the maximum likelihood estimator. I'm afraid the moral of the story is that there's many meanings of best in best estimator... Duncan === Subject: Re: Formula to combine physical measurements to one? Any book on Probability and Distribution Theory, I would have thought. Maybe see http://www.sph.umich.edu/syllabi/BIOSTAT601.pdf Nick === Subject: identity v tautology Does 'identity' and 'tautology' mean the same thing, or is there an essential difference between the two? === Subject: Re: identity v tautology Identity is the result of a tautology. ~v~~ === Subject: Re: identity v tautology No it does not mean the same thing, yes there's an essential difference between this two notions. F. -- === Subject: Re: identity v tautology See http://www.wittgenstein.internet-today.co.uk/plato.html The basic meaning of the term tautology in mathematics and philosophy is that a proposition is tautologous if it is true whatever the facts in the empirical world turn out to be. This is not the normal English meaning, but an example will show how the meanings relate: the tautology all stallions are horses. It is tautologous because of the meaning of its words, which preclude the possibility of an animal being a stallion but not a horse. Thus, an explorer who finds an adult male okapi will have to decide whether to refrain from calling it a stallion or to agree to calling an okapi a type of horse. So from the question of the meaning of words, we arrive at a situation where what might actually be found in the world must be considered. Nick === Subject: Re: identity v tautology Unfortunately this isn't the definition for anything anywhere in math, philosophy, or ordinary language because at best it just tells us when something is something and not what the something is. Didn't your fifth grade teacher ever tell you not to define things by the is when . . . technique because it's not exhaustive? A tautology is combination of alternatives exhaustive of the possibilities of truth as in A not A provided A is not self contradictory. A truism is problematic syllogistic inference as in If A then B. That's a proposition not a tautology. Says who? It would depend on the meaning of the terms. There aren't any pre emptive meanings to such words so the idea that ordinary propositions are exhaustive of the possibilities for truth is absurd. Ordinary propositions are either true, false, or problematic but they certainly don't exhaust the possibilities for truth. ~v~~ === Subject: Re: Simultaneous Equations Cc: deepkdeb@yahoo.com opinion on the following Consider (A-1) under the given conditions. x^2 + y^2 = z (A-1) My assertion: (A-1) can be satisfied only if: (1) both x and y are integers, (2) x = sqrt(m) and y = sqrt(n) where both m and n are not perfect squares (3) x = sqrt(m), y = sqrt(n) where only one of m or n is a perfect squre, (4) x = a + sqrt(b) and y = a - sqrt(b), b is not a perfect square. Is there any other conditions possible to make z an integer? Your reply will be appreciated. === Subject: Re: Simultaneous Equations You seem to keep missing the same fundamental concept. I'll phrase it this way. Free variables hate to be tied down. If you choose z, one of the variables x,y is still free. Thus, let z be any positive integer, and choose y such that (1) 0