Transcription of Variance and Standard Deviation - Penn Math
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Variance and Standard DeviationChristopher CrokeUniversity of PennsylvaniaMath 115 UPenn, Fall 2011 Christopher CrokeCalculus 115 VarianceThe first first important number describing a probabilitydistribution is the mean or expected valueE(X).The next one is thevarianceVar(X) = 2(X). The square root ofthe Variance is called theStandard (xi) is the probability distribution function for a randomvariable with range{x1,x2,x3,..}and mean =E(X) then:Var(X) = 2= (x1 )2f(x1) + (x2 )2f(x2) + (x3 )2f(x3) +..It is a description of how the distribution spreads .NoteVar(X) =E((X )2).The Standard Deviation has the same units asX.
wins but the second player gets $3 if she wins. No one gets payed if 4 white balls are chosen. We have seen that the payout and probabilities for the rst player are: Payout Probability 2 8 15 0 1 15 3 6 15 The expected value was = 2 15. What is the variance? Alternative formula for variance: ˙2 = E(X2) 2. Why? E((X )2) = E(X2 2 X + 2) = E(X2 ...
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