Results 1 to 7 of 7

Math Help - Transformation to improve normality

  1. #1
    Newbie
    Joined
    Jan 2013
    From
    uk
    Posts
    4

    Transformation to improve normality

    Hi, I have a problem with some data that I am trying to analyse in a Repeated Measures design.

    I have 5 conditions (variables), 13 subjects, data points ranging from about -40.0 to +40.0

    Both the Kolmogorov-Smirnov and Shapiro-Wilk tests are significant for some of the variables. Some of the variables are negatively skewed while others are positively skewed.

    I have tried log, sqrt, and inverse transforms (after adding a constant), however it depends on the direction of the skew as to whether I reflect them or not to get the best result.

    e.g. for the negatively skewed variable, i can improve normality by multiplying by -1, adding a constant to return the set to positive values, and then taking the log transform. However this makes the positively skewed values worse.

    I can't do different transforms on different variables and then do a repeated measures ANOVA.

    How can I address this problem?

    Thanks in advance for any assistance!
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    Joined
    Sep 2012
    From
    Australia
    Posts
    4,169
    Thanks
    765

    Re: Transformation to improve normality

    Hey rav8.

    Do you have to transform these? Have you considered non-parametric varieties of tests and statistics for your problem?
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Newbie
    Joined
    Jan 2013
    From
    uk
    Posts
    4

    Re: Transformation to improve normality

    Quote Originally Posted by chiro View Post
    Hey rav8.

    Do you have to transform these? Have you considered non-parametric varieties of tests and statistics for your problem?

    Hi, thanks for your reply. Yes, I could use non-parametric stats, however I always feel more comfortable using parametric - I know this isn't terribly rational! I'm not really a stats person at the best of times...

    If I don't have any luck transforming them, I won't have much choice I guess!
    Follow Math Help Forum on Facebook and Google+

  4. #4
    MHF Contributor
    Joined
    Sep 2012
    From
    Australia
    Posts
    4,169
    Thanks
    765

    Re: Transformation to improve normality

    What kind of applications, tests, and inferences did you have in mind?
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Newbie
    Joined
    Jan 2013
    From
    uk
    Posts
    4

    Re: Transformation to improve normality

    I just want to do a repeated measures anova with some posthocs...
    Follow Math Help Forum on Facebook and Google+

  6. #6
    MHF Contributor
    Joined
    Sep 2012
    From
    Australia
    Posts
    4,169
    Thanks
    765

    Re: Transformation to improve normality

    I took a look at repeated and it said that along with independent samples and normality, it also requires sphericity on top of this.

    This is just my opinion but I would look either look into non-parametric variants of the test, or if the normality and sphericity values are only just significant enough, then use the ANOVA with a much higher confidence level (instead of 95% make it say 97% or 98%).

    Make sure you check sphericity in your statistics package and if the two tests (sphericity and normality) are too significant, then you should be looking at non-parametric variants without question.
    Follow Math Help Forum on Facebook and Google+

  7. #7
    Newbie
    Joined
    Jan 2013
    From
    uk
    Posts
    4

    Re: Transformation to improve normality

    Quote Originally Posted by chiro View Post
    I took a look at repeated and it said that along with independent samples and normality, it also requires sphericity on top of this.

    This is just my opinion but I would look either look into non-parametric variants of the test, or if the normality and sphericity values are only just significant enough, then use the ANOVA with a much higher confidence level (instead of 95% make it say 97% or 98%).

    Make sure you check sphericity in your statistics package and if the two tests (sphericity and normality) are too significant, then you should be looking at non-parametric variants without question.

    Hi, thanks for your comments, sphericity is OK with this data - it's just the normality.

    Does anyone have any other transform suggestions? Otherwise I might have to go with non-parametric
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Improve the representation of the data
    Posted in the Statistics Forum
    Replies: 1
    Last Post: September 8th 2012, 10:02 PM
  2. Can I improve the accuracy of this method?
    Posted in the Advanced Statistics Forum
    Replies: 5
    Last Post: October 14th 2011, 09:50 AM
  3. Normality
    Posted in the Advanced Algebra Forum
    Replies: 1
    Last Post: February 22nd 2008, 08:35 AM
  4. Improve
    Posted in the Algebra Forum
    Replies: 3
    Last Post: January 25th 2008, 02:32 AM

Search Tags


/mathhelpforum @mathhelpforum