Dear Beatriz, There are several problems that combine to make the computation unstable: the small size of the numbers, the small ratio of the largest to the smallest value, the extreme negative skew of the data, and the outliers at the low end. Because of the small ratio of max(x) to min(x), power transformations are ineffective and an extreme power is required:
> max(x)/min(x) [1] 1.129898 > powerTransform(1e7*x) Estimated transformation parameters 1e+07 * x 31.77628 You can use a negative start to produce a more sensible result: > powerTransform(1e7*(x - .99*(min(x)))) Estimated transformation parameters 1e+07 * (x - 0.99 * (min(x))) 2.774309 I was slightly surprised that the magnitude of the numbers is an issue. I hope this helps, John ------------------------------------------------ John Fox Sen. William McMaster Prof. of Social Statistics Department of Sociology McMaster University Hamilton, Ontario, Canada http://socserv.mcmaster.ca/jfox/ On Thu, 31 Jan 2013 00:38:23 -0500 Beatriz González <aguitatie...@hotmail.com> wrote: > Hello, > > I would like to perform a Box-Cox (âbcPowerâ?) transformation on my data. > For this, I am determining lambda using the âpowerTransformâ? function. > However, with one of my variables I get the following > > Warning Message: > In estimateTransform(x, y, NULL, ...) : > Convergence failure: return code = 52 My variable is: > > x > [1] 0.0001031130 0.0001029480 0.0001040010 0.0001037940 0.0001046280 > 0.0000927650 0.0000942960 > [8] 0.0000950250 0.0000949780 0.0000988210 0.0000975860 0.0000962850 > 0.0001015800 0.0001009190 > [15] 0.0001035350 0.0001033210 0.0001028420 0.0001023150 0.0001033000 > 0.0001036910 0.0001026890 > [22] 0.0001032480 0.0001012160 0.0001031270 0.0001034390 0.0001028180 > 0.0001034820 0.0001022700 > [29] 0.0001042640 0.0001046720 0.0001045860 0.0001045160 0.0001045850 > 0.0001038040 0.0001037020 > [36] 0.0001034050 0.0001048150 0.0001017180 0.0001017920 0.0001032730 > 0.0001029420 0.0001025220 > [43] 0.0001031490 0.0001036995 0.0001039735 0.0001014877 0.0001027960 And my > code is:dlambda <- powerTransform(x) #It is here where I get the warning > message > x_BCx <- bcPower(x, lambda= dlambda$lambda, jacobian.adjusted= FALSE) > Does anyone know why I may be getting this Warning Message? If you require > more details please get in touch. > > Many thanks in advance! > > Bea > [[alternative HTML version deleted]] > ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.