Lognormal Distribution

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Lognormal Distribution
Message posted by Paul on December 1, 1999 at 12:00 AM (ET)

I have historical data for variables D and P from which I calculate Y=D/P and the historical average of Y, Yavg. D and P are always positive numbers.
Given the current value of Y, I want to determine the current Y𠏋 statistical deviation from the historical Yavg. Y is lognormal, so I calculate the statistics for the natural logarithm of the historical Y. Then I calculate Z for the current LnY.

If Z = 2, is it valid to say that Y is statistically far from Yavg because LnY is two standard deviations from the average of the historical LnY𠏋.?

I am concerned about changing the weighting factor on the data by the logarithm transformation. I am interested in Y, not the Ln of Y. Is this a valid concern?

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Re: Lognormal Distribution
Message posted by SOF on December 2, 1999 at 12:00 AM (ET)

M. J. Moroney in 'facts from figure' on page 139 suggests that a lognormal distribution can be made approximately normal by a logarithmic transformation. After you finish your analysis you would use a antilogarithmic transformation to translate your numbers back to the original data.

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