Binomial Distribution - Lecture Notes | STAT 205, Study notes of Statistics

Material Type: Notes; Professor: Hendrix; Class: ELEM STATS/BIOL&LIFE SCI; Subject: Statistics; University: University of South Carolina - Columbia; Term: Fall 2009;

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Uploaded on 10/01/2009

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Binomial๎˜ƒDistribution๎˜ƒ
Letโ€™s๎˜ƒrefresh๎˜ƒour๎˜ƒmemory๎˜ƒon๎˜ƒcomputing๎˜ƒprobability๎˜ƒwith๎˜ƒan๎˜ƒexample.๎˜ƒ๎˜ƒConsider๎˜ƒthe๎˜ƒexperiment๎˜ƒ
where๎˜ƒwe๎˜ƒtoss๎˜ƒa๎˜ƒfair๎˜ƒcoin๎˜ƒ3๎˜ƒtimes.๎˜ƒ๎˜ƒFind๎˜ƒthe๎˜ƒprobability๎˜ƒdistribution๎˜ƒfor๎˜ƒflipping๎˜ƒโ€œheadsโ€๎˜ƒin๎˜ƒthis๎˜ƒ
experiment.๎˜ƒ๎˜ƒ๎˜ƒ
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Now,๎˜ƒthink๎˜ƒabout๎˜ƒfinding๎˜ƒprobability๎˜ƒdistributions๎˜ƒassociated๎˜ƒwith๎˜ƒflipping๎˜ƒa๎˜ƒfair๎˜ƒcoin๎˜ƒsay๎˜ƒ6๎˜ƒ
times.๎˜ƒ๎˜ƒAnd๎˜ƒthen๎˜ƒconsider๎˜ƒthe๎˜ƒexperiment๎˜ƒwhere๎˜ƒthe๎˜ƒcoin๎˜ƒis๎˜ƒnot๎˜ƒfair.๎˜ƒ๎˜ƒThe๎˜ƒcalculations๎˜ƒget๎˜ƒ
unwieldy๎˜ƒfast!๎˜ƒ๎˜ƒWe๎˜ƒneed๎˜ƒa๎˜ƒmore๎˜ƒconvenient๎˜ƒmethodโ€ฆ๎˜ƒ
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Binomial Distribution

Letโ€™s refresh our memory on computing probability with an example. Consider the experiment where we toss a fair coin 3 times. Find the probability distribution for flipping โ€œheadsโ€ in this experiment. Now, think about finding probability distributions associated with flipping a fair coin say 6 times. And then consider the experiment where the coin is not fair. The calculations get unwieldy fast! We need a more convenient methodโ€ฆ

Binomial Distribution Page 2 Definition: The independent trials model occurs when (i) n independent trials are studied (ii) each trial results in a single binary observation (iii) each trialโ€™s success has (constant) probability: P{success} = p Notice that if P{success} = p, P{failure} = 1โ€“p. Your text calls this the BInS (Binary / Indep. / n is constant / Same p) setting, but is commonly referred to as a Binomial Experiment In a BInS setting, if we let Y = {# successes} then Y has a binomial distribution. NOTATION: Y ~ Bin(n,p). The binomial probability function is P{Y = j} = (^) n Cj p j (1 โ€“ p) nโ€“j j = 0,1,โ€ฆ,n where (^) n Cj = n! j!เตซnโ€jเตฏ! with j! = j(jโ€1)(jโ€2)โ€ฆ(2)(1) and define 0! = 1 Example Use the binomial probability function to find P{exactly 1 head} in the experiment where a fair coin is flipped 3 times. Find P{at least one head}