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Normdist Problem
I put in the following:
X=-.055762 Mean=.005 SD=.04 Cumulative=FALSE =means probability mass function, not cumulative The result displayed is 3.14615427 The result makes no sense to me. Any result over one makes no sense to me. I reviewed the answer from a previous thread (Jerry W. Lewis, 8/3/2006) that continuous functions do not have a probability mass, but rather a pdf, an integral over a tiny range around -.055, but the Excel function result still makes no sense. What am I missing here? |
#2
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Normdist Problem
With cumulative=FALSE, aren't you looking at a density function? In other
words if the cumulative probability is P, you are looking at the differential dP/dx. Because you have a very small value of SD, the value of P is going from 0 to 1 across a very narrow range of x values, so the gradient is very large (and can happily go beyond 1). It is the *cumulative* probability that can't exceed 1, not the probability density. You can easily check the calculation of the probability density function. If X is in A1, mean in A2, and SD in A3, the density is =(1/(SQRT(2*PI())*A3))*EXP(-((A1-A2)^2/(2*A3^2))) You'll see the formula in Excel help for NORMDIST, or in any stats textbook. -- David Biddulph Rodby wrote: I put in the following: X=-.055762 Mean=.005 SD=.04 Cumulative=FALSE =means probability mass function, not cumulative The result displayed is 3.14615427 The result makes no sense to me. Any result over one makes no sense to me. I reviewed the answer from a previous thread (Jerry W. Lewis, 8/3/2006) that continuous functions do not have a probability mass, but rather a pdf, an integral over a tiny range around -.055, but the Excel function result still makes no sense. What am I missing here? |
#3
Posted to microsoft.public.excel.worksheet.functions
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Normdist Problem
With cumulative=FALSE, aren't you looking at a density function? In other
words if the cumulative probability is P, you are looking at the differential dP/dx. Because you have a very small value of SD, the value of P is going from 0 to 1 across a very narrow range of x values, so the gradient is very large (and can happily go beyond 1). It is the *cumulative* probability that can't exceed 1, not the probability density. You can easily check the calculation of the probability density function. If X is in A1, mean in A2, and SD in A3, the density is =(1/(SQRT(2*PI())*A3))*EXP(-((A1-A2)^2/(2*A3^2))) You'll see the formula in Excel help for NORMDIST, or in any stats textbook. -- David Biddulph Rodby wrote: I put in the following: X=-.055762 Mean=.005 SD=.04 Cumulative=FALSE =means probability mass function, not cumulative The result displayed is 3.14615427 The result makes no sense to me. Any result over one makes no sense to me. I reviewed the answer from a previous thread (Jerry W. Lewis, 8/3/2006) that continuous functions do not have a probability mass, but rather a pdf, an integral over a tiny range around -.055, but the Excel function result still makes no sense. What am I missing here? |
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