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/ . =================================================================
