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Material Type: Notes; Professor: Hendrix; Class: ELEM STATS/BIOL&LIFE SCI; Subject: Statistics; University: University of South Carolina - Columbia; Term: Fall 2009;
Typology: Study notes
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Letโs 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 โheadsโ 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 methodโฆ
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 trialโs success has (constant) probability: P{success} = p Notice that if P{success} = p, P{failure} = 1โp. 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) nโj j = 0,1,โฆ,n where (^) n Cj = n! j!เตซnโjเตฏ! with j! = j(jโ1)(jโ2)โฆ(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}