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Lectures 20/21 Poisson distribution As a limit to binomial when n is large and p is small. A theorem by Simeon Denis Poisson(1781-1840). Parameter = np= expected value As n is large and p is small, the binomial probability can be approximated by the Poisson probability function P(X=x)= e - x / x!, where e =2.71828 Ion channel modeling : n=number of channels in cells and p is probability of opening for each channel;

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Binomial and Poisson approximation xn=100, p=.01Poisson 0.366032.367879 1.36973.367879 2.184865.183940 3.06099.061313 4.014942.015328 5.002898.003066 6.0000463.000511 7

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Advantage: No need to know n and p; estimate the parameter from data X= Number of deathsfrequencies 0109 165 222 33 41 total200 200 yearly reports of death by horse-kick from10 cavalry corps over a period of 20 years in 19th century by Prussian officials.

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xData frequencies Poisson probability Expected frequencies 0109.5435108.7 165.331566.3 222.10120.2 33.02054.1 41.0030.6 200 Pool the last two cells and conduct a chi-square test to see if Poisson model is compatible with data or not. Degree of freedom is 4-1-1 = 2. Pearson’s statistic =.304; P-value is.859 (you can only tell it is between.95 and.2 from table in the book); accept null hypothesis, data compatible with model

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Rutherfold and Geiger (1910) Polonium source placed a short distance from a small screen. For each of 2608 eighth-minute intervals, they recorded the number of alpha particles impinging on the screen Medical Imaging : X-ray, PET scan (positron emission tomography), MRI Other related application in

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# of particles Observed frequencyExpected freq. 05754 1203211 2383407 3525526 4532508 5408394 6273254 7139140 84568 92729 10 11 11+66

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Pearson’s chi-squared statistics = 12.955; d.f.=12-1-1=10 Poisson parameter = 3.87, P-value between.95 and.975. Accept null hypothesis : data are compatible with Poisson model

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Poisson process for modeling number of event occurrences in a spatial or temporal domain Homogeneity : rate of occurrence is uniform Independent occurrence in non- overlapping areas Non-clumping

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01234+total Obs.58439816835151200 Exp.561.6427.2162.040.88.41200 =.76 (= [398+ 2(168) + 3(35)+4(9)+5(4)+6(0)+7(1)+9(1)]/1200 Pearson’s statistic=9.12, d.f=3, P-value is0.0277, reject Poisson model at 5% level; but accept at 1% level. Village housing pattern. West of Tokyo lies a large alluvial plain. Position of 911 houses are recorded in a grid which divides the area into 1200 plots in a 30 by 40 matrix as shown below.

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