You might use something like a Q-Q plot to see which gives you the closest to a normal distribution. David Howell's "big" textbook shows you how.
Chris ....... Christopher D Green Department of Psychology York University Toronto, ON M6C 1G4 [email protected] http://www.yorku.ca/christo > On Nov 11, 2013, at 7:26 PM, Michael Britt <[email protected]> wrote: > > Thanks for all these suggestions. I've been thinking of trying all or most > of them. I assume that I would get slightly different results, so let me ask > this: what criteria would you use to determine which transformation gave the > best result? Would it be the one with the least amount of skew/most normal > in distribution? > > > Michael A. Britt, Ph.D. > [email protected] > http://www.ThePsychFiles.com > Twitter: @mbritt > >> On Nov 11, 2013, at 7:04 PM, Jim Clark <[email protected]> wrote: >> >> Hi >> >> People have been noting various transformations, depending on reversing or >> not the original. In essence many common transformations can be >> conceptualized as the original scores raised to various powers, with the >> powers being greater than or less than 1. >> >> x^-1 = reciprocal 1/x >> x^~0 = logarithmic >> x^.5 = square root >> x^1 = original >> x^2 = ... >> >> I'm mostly used to thinking of these in terms of various non-linear >> relationships (powers < 1 compress upper end, powers > 1 expand upper end), >> but some of these will increase skewness and others will decrease it, >> depending on the direction of skew. Possible to experiment with them to >> observe effect. >> >> Perhaps also worth plotting some of the relationships you are interested in? >> >> Take care >> Jim >> >> Jim Clark >> Professor & Chair of Psychology >> 204-786-9757 >> 4L41A >> >> -----Original Message----- >> From: Christopher Green [mailto:[email protected]] >> Sent: Monday, November 11, 2013 5:31 PM >> To: Teaching in the Psychological Sciences (TIPS) >> Subject: Re: [tips] What to do with skewed data >> >> Michael, >> >> There are couple of standard ways to transform skewed data. Invert the data >> (subtract each datum from one greater than the highest value) so that the >> skew is positive. Then, Depending of the strength of the skew, do a square >> root or logarithmic transformation. Alternatively, don't invert it and take >> the reciprocal (1/x) of each datum). >> >> Chris >> ....... >> Christopher D Green >> Department of Psychology >> York University >> Toronto, ON M6C 1G4 >> >> [email protected] >> http://www.yorku.ca/christo >> >>> On Nov 11, 2013, at 1:01 PM, Michael Britt <[email protected]> >>> wrote: >>> >>> I did a survey which asked respondents how satisfied they are in their >>> current (romantic) relationship on a 1=10 point scale (where 10="very >>> satisfied). While there was some variation, not surprisingly, the results >>> are strongly negatively skewed. That makes sense - most people are >>> probably satisfied with their relationships or they would leave the other >>> person (or there's some form of cognitive dissonance going on, but that's >>> not my question. >>> >>> No matter how big the sample size (mine was 160 respondents) I assume >>> you'll always get a skewed distribution on a question like this so wouldn't >>> I be breaking the normalization assumption if I were to do correlations >>> using these results? I assume I could either do: a) do some kind of >>> transformation - but I've never done one before so I’m not familiar with >>> it, or b) recode the data into 3 categories (perhaps 1-5 is low >>> satisfaction, 6-7 is moderate and 8-10 is high) and do a chi-squre instead >>> of a correlation. >>> >>> Any thoughts? Appreciate it. >>> >>> Michael >>> >>> Michael A. Britt, Ph.D. >>> [email protected] >>> http://www.ThePsychFiles.com >>> Twitter: @mbritt >>> >>> >>> --- >>> You are currently subscribed to tips as: [email protected]. >>> To unsubscribe click here: >>> http://fsulist.frostburg.edu/u?id=430248.781165b5ef80a3cd2b14721caf62b >>> d92&n=T&l=tips&o=30023 or send a blank email to >>> leave-30023-430248.781165b5ef80a3cd2b14721caf62bd92@fsulist.frostburg. >>> edu >> >> --- >> You are currently subscribed to tips as: [email protected]. >> To unsubscribe click here: >> http://fsulist.frostburg.edu/u?id=13251.645f86b5cec4da0a56ffea7a891720c9&n=T&l=tips&o=30040 >> or send a blank email to >> leave-30040-13251.645f86b5cec4da0a56ffea7a89172...@fsulist.frostburg.edu >> >> --- >> You are currently subscribed to tips as: [email protected]. >> To unsubscribe click here: >> http://fsulist.frostburg.edu/u?id=13405.0125141592fa9ededc665c55d9958f69&n=T&l=tips&o=30042 >> or send a blank email to >> leave-30042-13405.0125141592fa9ededc665c55d9958...@fsulist.frostburg.edu > > > --- > You are currently subscribed to tips as: [email protected]. > To unsubscribe click here: > http://fsulist.frostburg.edu/u?id=430248.781165b5ef80a3cd2b14721caf62bd92&n=T&l=tips&o=30043 > or send a blank email to > leave-30043-430248.781165b5ef80a3cd2b14721caf62b...@fsulist.frostburg.edu > --- You are currently subscribed to tips as: [email protected]. 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