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Introduction to Distributions and Probability

Statistics for Health Research. Introduction to Distributions and Probability. Peter T. Donnan Professor of Epidemiology and Biostatistics. Overview. Distributions History of probability Definitions of probability Random variable Probability density function

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Introduction to Distributions and Probability

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  1. Statistics for Health Research Introduction to Distributions and Probability Peter T. Donnan Professor of Epidemiology and Biostatistics

  2. Overview Distributions History of probability Definitions of probability Random variable Probability density function Normal, Binomial and Poisson distributions

  3. Introduction to Probability Density Functions • Normal Distribution / • Gaussian / Bell curve • Poisson named after French Mathematician • Binomial related to binary factors (Bernoulli Trials)

  4. Early use of Normal Distribution Gauss was a German mathematician who solved mystery of where Ceres would appear after it disappeared behind the Sun. He assumed the errors formed a Normal distribution and managed to accurately predict the orbit of Ceres

  5. What is the relationship between the Normal or Gaussian distribution and probability?

  6. Probability “I cannot believe that God plays dice with the cosmos” Albert Einstein “The probable is what usually happens” Aristotle

  7. Origins of Probability Early interest in permutations Vedic literature 400 BC Distinguished origins in betting and gambling! Pascal and Fermat studied division of stakes in gambling (1654) Enlightenment – seen as helping public policy, social equity Astronomy – Gauss (1801) Social and genetic – Galton (1885) Experimental design – Fisher (1936)

  8. Types of Probability Two basic definitions: 1) Frequentist Classical Proportion of times an event occurs in a long series of ‘trials’ 2) Subjectivist Bayesian Strength of belief in event happening

  9. Frequentists vs. Bayesians Two entrenched camps Scientists tend to use the frequentist approach Bayesians gaining ground Most scientists use frequentist methods but incorrectly interpret results in a Bayesian way!

  10. Frequentists Consider tossing a fair coin In any trial, event may be a ‘head’ or ‘tail’ i.e. binary Repeated tossing gives series of ‘events’ In long run prob of heads=0.5 THTTHHHHTHHHTHHHTTHTTTHHTTHTTHHHTTTHHTHHHTTTTTHHH 0.6 0.56 0.52

  11. Frequentist Probability Note the difference between ‘long run’ probability and an individual trial In an individual trial a head either occurs (X=1) or does not occur (X=0) Patient either survives or dies following an MI Prob of dying after MI ≈ 30% based on a previous long series from a population of individuals who experienced MI

  12. Subjective Probability Based on strength of belief But more akin to thinking of clinician making a diagnosis Faced with patient with chest pain, based on past experience, believes prob of heart disease is 20% Person tossing coin believes prob of head is 1/2

  13. Comparison of definitions of Probability Problems of subjective probability Probability for same patient can vary even with same clinician Person can believeprob of head is 0.1 even if it is a fair coin Subjectivists argue they are more realistic This course sticks to ‘frequentist’ and ‘model-based’ methods of probability

  14. Random Variable Consider rolling 2 dice and we want to summarise the probabilities of all possible outcomes We call the outcome a random variable X which can have any value in this case from 2 to 12 Enumerate all probabilities in sample space S P (2) = 1/6x1/6 = 1/36, P (3)=2/36, P (4) = 3/36, etc…..

  15. Probability Density Function for rolling two dice 6/36 5/36 4/36 3/36 2/36 1/36 1/36 2/36 3/36 4/36 5/36 6/36 5/36 4/36 3/36 2/36 1/36 2 3 4 5 6 7 8 9 10 11 12 2 3 4 5 6 7 8 9 10 11 12

  16. Probability Density Function for rolling two dice 6/36 5/36 4/36 3/36 2/36 1/36 2 3 4 5 6 7 8 9 10 11 12 What is probability of getting 12? Answer 1/36 What is probability of getting more than 8? Ans. 10/36

  17. Probability Density Function for continuous variable 6/36 5/36 4/36 3/36 2/36 1/36 1/36 2/36 3/36 4/36 5/36 6/36 5/36 4/36 3/36 2/36 1/36 2 3 4 5 6 7 8 9 10 11 12 2 3 4 5 6 7 8 9 10 11 12

  18. Consider distribution of weight in kg; all values possible not just discrete Probability 20…….30……40…… 50 ……60…….70…….80…..90….100….110…… 120 2 3 4 5 6 7 8 9 10 11 12 Weight in kilograms

  19. Probability Density Function in SPSS Use Analyze / Descriptive Statistics / Frequencies and select no table and charts box as below

  20. Probability Density Function in SPSS Data from ‘LDL Data.sav’ of baseline LDL cholesterol

  21. Normal Distribution Note that a Normal or Gaussian curve is defined by two parameters: Mean µ and Standard Deviation σ And often written as N ( µ, σ ) Hence any Normal distribution has mathematical form Impossible to be integrated so area under the curve obtained by numerical integration and tabulated!

  22. Normal Distribution As noted earlier the curve is symmetrical about the mean and so p ( x ) > mean = 0.5 or 50% And p ( x ) < mean = 0.5 or 50% And p (a < x < b) = p(b) – p(a) 50% 50%

  23. Normal Distribution and Probabilities So we now have a way of working out the probability of any value or range of values of a variables IF a Normal distribution is a reasonable fit to the data p (a < x < b) = p(b) – p(a) which is the area under the curve between a and b 50% 50%

  24. Normal Distribution Most of area lies between +1 and -1 SD (64%) The large majority lie between +2 and -2 SDs (95%)

  25. Normal Distribution Probability Density Function (PDF) =

  26. How well does my data fit a Normal Distribution? Note median and mean virtually the same Skewness = 0.039, close to zero Skewness is measure of symmetry (0=perfect symetry) Eyeball test - fitted normal curve looks good!

  27. Try Q-Q plot in Analyze / Descriptive Statistics/ Q-Q plot Plot compares Expected Normal distribution with real data and if data lies on line y = x then the Normal Distribution is a good fit Note still an eyeball test! Is this a good fit?

  28. I used to be Normal until I discovered Kilmogorov-Smirnoff! Eyeball Test indicates distribution is approximately Normal butK-S test is significant indicating discrepancy compared to Normal WARNING: DO NOT RELY ON THIS TEST

  29. Consider the distribution of survival times following surgery for colorectal cancer Note median=835 days and mean=848 Skewness = 2.081, very skewed (> 1.0) Strong tail to right! Approximately Normal?

  30. Try a log transformation for right positive skewed data? Better but now slightly skewed to left!

  31. Examples of skewed distributions in Health Research Discrete random variables – hospital admissions, cigarettes smoked, alcohol consumption, costs Continuous RV – BMI, cholesterol, BP 30%

  32. The Binomial Distribution ‘Binomial’ means ‘two numbers’. Outcomes of health research are often measured by whether they have occurred or not. For example, recovered from disease, admitted to hospital, died, etc May be modelled by assuming that the number of events n has a binomial distribution with a fixed probability of event p

  33. The Binomial Distribution Based on work of Jakob Bernoulli, a Swiss mathematician Refused a church appointment and instead studied mathematics Early use was for games of chance but now used in every human endeavour When n = 1 this is called a Bernoulli trial Binomial distribution is distribution for a series of Bernoulli trials

  34. The Binomial Distribution Binomial distribution written as B ( n , p) where n is the total number of events and p = prob of an event This is a Binomial Distribution with p=0.25 and n=20

  35. The Binomial Distribution

  36. The Poisson Distribution Poisson distribution (1838), named after its inventor Simeon Poisson who was a French mathematician. He found that if we have a rare event (i.e. p is small) and we know the expected or mean ( or µ) number of occurrences, the probabilities of 0, 1, 2 ... events are given by:

  37. The Poisson Distribution Note similarity to Binomial In fact when p is small and n is large B(n, p) ~ P (µ = np) Also for large values of µ: P (µ) ~ N ( µ, µ ) Hence if n and p not known could use Poisson instead

  38. The Poisson Distribution In health research often used to model the number of events assumed to be random: Number of hip replacement failures, Number of cases of C. diff infection, Diagnoses of leukaemia around nuclear power stations, Number of H1N1 cases in Scotland, Etc.

  39. Summary • Many of variables measured in Health Research form distributions which approximate to common distributions with known mathematical properties • Normal, Poisson, Binomial, etc… • Note a relationship for all centred • around the exponential distribution • Where e = 2.718 • All belong to the Exponential Family of distributions • These probability distributions are critical to applying statistical methods

  40. SPSS Practical Read in data file ‘LDL Data.sav’ Consider adherence to statins, baseline LDL, min Chol achieved, BMI, duration of statin use Assess distributions for normality If non-normal consider a transformation Try to carry out Q-Q plots

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