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From | Steve Samuels <sjsamuels@gmail.com> |
To | statalist@hsphsun2.harvard.edu |
Subject | Re: st: Why many things have Normal distribution |
Date | Fri, 30 Aug 2013 18:12:02 -0400 |
David, Here is some empirical evidence: the book by Hampel et al.(1986, pp 22-23) cites several investigators, starting with Bessel in 1818, who studied "very high quality" data sets. Most of the sets were longer-tailed than the normal and were well-approximated by t-distributions with 3-9 d.f. Slight skewness was also noted. Steve Reference: Hampel, Frank, Elvezio Ronchetti, Peter Rousseeuw, and Werner Stahel. 1986. Robust Statistics: The Approach Based on Influence Functions (Wiley Series in Probability and Mathematical Statistics). New York: John Wiley and Sons. Jeffereys, H. (1939,1961). Theory of Probability. Clarendon Press, Oxford On Aug 29, 2013, at 10:49 PM, David Hoaglin wrote: Yuval, The Central Limit Theorem (CLT) describes the behavior of the distribution of the sample mean as the sample size becomes large. In order for the distribution of the sample mean to approach a normal distribution, the underlying distribution of the data must satisfy some conditions, but those conditions are not very stringent. The CLT provides no information on how the underlying distribution behaves. One does, however, expect the behavior of samples to approach that of the underlying distribution (whatever that happens to be). I would have no special expectations of the distribution of heights in a large army. I would look at the actual distribution --- empirical evidence, rather than a thought experiment. Apart from any attempts to avoid serving, one would expect recruiters to reject people who were too short and people who were too tall. Also the actual distribution might be a mixture of components. As I recall, in the 19th century Quetelet used a frequency distribution of the chest circumference of Scottish soldiers to illustrate a method of fitting a normal distribution. In compiling the data he merged several components and made a variety of mistakes. The outcomes of tossing an actual "fair" die depend on how carefully the die was manufactured. Iversen et al. (1971) analyzed the results of a large number of throws of various types of dice. You didn't say how you would use a normal distribution to approximate the outcomes of throwing a fair die. The basic distribution is discrete, with six equally likely outcomes. David Hoaglin Iversen GR, Longcor WH, Mosteller F, Gilbert JP, Youtz C (1971). Bias and runs in dice throwing and recording: a few million throws. Psychometrika 36:1-19. On Thu, Aug 29, 2013 at 5:38 PM, Yuval Arbel <yuval.arbel@gmail.com> wrote: > What about the central limit theorem? I was referring to physical > human features - such as height - and the example of Napoleon's army > candidates for draft. In an army of millions of soldiers - you would > expect a normal distribution of heights. The problem is that those who > tried to avoid drafting probably bribed somebody to write false > heights, which is shorter than the minimal required height. In this > case - you might get a skewed distribution of heights * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/faqs/resources/statalist-faq/ * http://www.ats.ucla.edu/stat/stata/ * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/faqs/resources/statalist-faq/ * http://www.ats.ucla.edu/stat/stata/