At the risk of sounding totally ignorant, what is a "lamba transform," or where could I read about it? DW --- Jay Warner <[EMAIL PROTECTED]> wrote: > No, and maybe yes. > > What you are doing is transforming the x axis so > that the resultant > distribution appears as a Normal. If it works :) > > A 'positively skewed' distribution is not > infrequently something like > a log-Normal. Thus, a straight log of the x values > gives a nice > Normal upon transformation. > > Look at the picture, already! (sorry. I get > frustrated...) > > the negatively skewed distribution can be > transformed also. Either > 'flipping it, as you suggest, or perhaps by using > Taguchi's 'omega' > transform. (It has another name in 'western' > literature, but I don't' > know what - arc-sine?) > > x' = ln(x/(w-x)) > > where x is the original x value, w is a value > slightly in excess of > the highest possible x. It could be ln or log, > either way, and x' is > the new x scale value. > > this works nicely, very nicely. You can get a shift > in the resultant > distribution, by adjusting w if you like. > Interpretation leaves > something to be desired..... > > If the x scale values are less than 0, you need to > be careful to > ensure that the ln is performed on positive values, > is all. > > If you can justify the transformation based on the > technology you are > playing with, so much the better. If you justify it > on the basis that > the resultant distribution is 'more' Normal, you > might as well go for > a lambda transform, and be done with it. > > Cheers, > Jay > > Dianne Worth wrote: > > > It is my understanding that if the data > distribution is positively > > skewed , one can take a log of the data. If it's > negatively skewed, > > one should 'flip' it by adding 1 to the largest > value, and a series > > of steps that are probably best explained as > follows:Original values > > are 1-8.Add 1 to 8 = > 9.FLIPX=9-XLOGFLIPX=LOG(FLIPX)LOGX=.903089 - > > LOGFLIPXwhere .903089 is the log of the original > largest value > > (8). My question: What if my positively skewed > data all have > > negative values, say from -1 to -7? Does the same > logic/procedure > > still hold?Thanks,DW > > > --------------------------------------------------------------------- > > Do you Yahoo!? > > SBC Yahoo! DSL - Now only $29.95 per month! > > -- > Jay Warner > Principal Scientist > Warner Consulting, Inc. > 4444 North Green Bay Road > Racine, WI 53404-1216 > USA > > Ph: (262) 634-9100 > FAX: (262) 681-1133 > email: [EMAIL PROTECTED] > web: http://www.a2q.com > > The A2Q Method (tm) -- What do you want to improve > today? > > > > > . > . > ================================================================= > Instructions for joining and leaving this list, > remarks about the > problem of INAPPROPRIATE MESSAGES, and archives are > available at: > . http://jse.stat.ncsu.edu/ > . > =================================================================
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