M249 Practical modern statistics Bayesian statistics About this module M249 Practical modern statistics uses the software packages IBM SPSS Statistics (SPSS Inc.) and WinBUGS, and other software. This software is provided as part of the module, and its use is covered in the Introduction to statistical modelling and in the four computer books associated with Books 1 to 4. This publication forms part of an Open University module. Details of this and other Open University modules can be obtained from the Student Registration and Enquiry Service, The Open University, PO Box 197, Milton Keynes MK7 6BJ, United Kingdom (tel. +44 (0)845 300 60 90; email [email protected]). Alternatively, you may visit the Open University website at www.open.ac.uk where you can learn more about the wide range of modules and packs offered at all levels by The Open University. 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ISBN 978 0 7492 1369 5 1.1 Contents The Bayesian approach 1 Introduction 1 1 Estimating probabilities 2 2 Bayes’ theorem 6 2.1 Conditional probability 6 2.2 Bayes’ theorem 9 2.3 Sequential updating 12 3 A framework for Bayesian inference 16 3.1 Prior distributions 16 3.2 Conditional distributions 19 3.3 The likelihood function 20 3.4 Posterior distributions 23 Solutions to Activities 28 Solutions to Exercises 31 Index 33 The Bayesian approach Introduction One of the key ideas in Bayesian statistics is that knowledge about anything unknown can be expressed probabilistically. For example, suppose we are interested in the proposition E, where A proposition is any kind of statement which can be true or E = a major earthquake will occur in Europe in the next ten years. false. Since E is unobserved, there is uncertainty as to whether or not E is true. In Bayesian statistics, knowledge about E can be expressed by P (E), the probability that E is true. The closer the value of P (E) is to 1, the more likely E is to be true, and the closer P (E) is to 0, the more likely E is to be false. In a Bayesian statistical analysis, probabilities such as P (E), which represent knowledge about E, need to be estimated. How this might be done is the focus of Section 1. Suppose that data are observed, or new information becomes available, to help learn about a proposition. The data (or information) will update the knowledge about the proposition. This updated knowledge can be expressed probabilistically. In Section 2, Bayes’ theorem is introduced. This provides a method of calculating the probability representing the updated knowledge. In most statistical problems, it is not simply the probability of a proposition that is of interest, but the more general problem of learning about an unknown parameter or, indeed, any unobserved quantity. In Bayesian statistics, knowledge about an unknown parameter, or quantity, can also be expressed probabilistically. Bayes’ theorem provides a method of calculating the updated knowledge about the unknown parameter or quantity, as represented probabilistically, after observing any relevant data or information. How this can be done is discussed in Section 3, and a framework for Bayesian inference is established. 1 1 Estimating probabilities Often in Bayesian statistics the probability of some proposition has to be estimated. In this section, several ways in which this might be done are discussed. These include simply calculating probabilities and using relative frequencies to estimate probabilities. You will be familiar with these methods from classical statistics. Probabilities that cannot be estimated objectively and need to be estimated subjectively are also considered. Calculating probabilities Some probabilities can simply be calculated, as illustrated in Example 1.1. Example 1.1 Tossing a coin If a coin is tossed, then assuming that the coin is fair, the probability that it lands heads up is calculated as number of ways of getting a head 1 P (head) = = . number of possible outcomes (head or tail) 2 Activity 1.1 Rolling a die A fair die with faces numbered from 1 to 6 is rolled. Calculate the probability that the die lands with an even number uppermost. In Example 1.1 and Activity 1.1, there is little uncertainty about the values of the probabilities, and it is unlikely there would be disagreement about the values. Estimating probabilities using relative frequencies Another method of estimating the probability of an event is to use the observed, or hypothetical, relative frequency of the event. The relative frequency of an event is the number of occurrences of the event divided by the number of times the event could have occurred. This is illustrated in Examples 1.2 to 1.4. Example 1.2 Probability of Down’s syndrome In every pregnancy there is a risk that the foetus has Down’s syndrome. The risk increases with maternal age, so an older mother is more likely to be carrying a foetus with Down’s syndrome than is a younger mother. Several large-scale surveys have been carried out to estimate, for different ages of the mother, the probability that a foetus has Down’s syndrome. For each maternal age, data are available on the proportion of foetuses with Down’s syndrome. These data provide a direct estimate of the relative frequency, and hence of the probability, of Down’s syndrome for different ages of the mother. These data were obtained in Table 1.1 (overleaf) shows estimates of P (Down’s), the probability of Down’s June 2004 from the website of syndrome at 40 weeks’ gestation, for different maternal ages. the Fetal Medicine Foundation http://www.fetalmedicine.com. 2 Section 1 Estimating probabilities Table 1.1 Estimated probability that a foetus has Down’s syndrome, by maternal age Maternal age (years) Estimate of P (Down’s) 20 1/1527 25 1/1352 30 1/895 31 1/776 32 1/659 33 1/547 34 1/446 35 1/356 36 1/280 37 1/218 38 1/167 39 1/128 40 1/97 41 1/73 42 1/55 43 1/41 44 1/30 45 1/23 Note that the probabilities in Table 1.1 are estimates and may not be equal to the true, underlying values of the probabilities. However, they are estimated from large amounts of data, so they should be very good estimates. Example 1.3 Probability of a volcanic eruption In January 1993, a conference on predicting volcanic eruptions was held at the Aspinall, W.P., Woo, G., foot of the Galeras volcano in Colombia. The highlight of the conference was to Voight, B., Baxter, P.J. et al. be a trip into the crater. After much debate concerning the risk (that is, the (2003) Evidence-based volcanology: application to probability) of Galeras erupting, the trip went ahead. However, Galeras erupted eruption crises. Journal of during the trip, killing six scientists and three tourists. Volcanology and Geothermal Research, 128, 273–285. Suppose that prior to the trip into the crater, an estimate was required of the probability of an imminent eruption, where ‘imminent’ is taken to mean within the next seven days.