Rather than create an attachment, I'm providing a link to a handout for my stats class. It portrays how one could have a positive relationship in one group and a negative relationship in another group, but overall there would be a positive relationship. If you simply imagine the upper group (x values from 6-10) as having a negative relationship, you would have one way that two groups with negative relationships could combine to produce a positive relationship. In a similar fashion, you could envision how two groups with positive relationships could combine to produce a negative relationship.
http://www.skidmore.edu/~hfoley/Handouts/S.Ch16-17.pdf [scroll down to the top of page 10] Hugh On Feb 27, 2012, at 11:30 AM, Helweg-Larsen, Marie wrote: I have a simple statistical question. I have a sample of 307 people. 111 are in the red group and 196 are the blue group. The correlation between variables x and y in the red group is r= .226 (n=111), p <.05 and in the blue group r=.164 (n=196), p<.05. However, when I run the correlation between x and y in the entire sample (red and blue combined, no missing data) I get a negative correlation, r=-.142 (n=307), p < .05. Now what doesn’t make sense to me that two groups individually have positive and significant correlations but the two groups combined can have a negative and significant correlation. So you stats tipsters. Is that statistically possible? I have checked everything I possibly can in terms of errors in the data or the analyses and have found none. Some suggestions about what I ought to look at? Marie Marie Helweg-Larsen, Ph.D. Associate Professor l Department of Psychology Kaufman 168 l Dickinson College Phone 717.245.1562 l Fax 717.245.1971 Office Hours: Mondays and Tuesdays 2:00-3:30 http://users.dickinson.edu/~helwegm/index.html --- You are currently subscribed to tips as: [email protected]<mailto:[email protected]>. To unsubscribe click here: http://fsulist.frostburg.edu/u?id=13238.0e762b65028402721e10bbc97ede52b7&n=T&l=tips&o=16294 (It may be necessary to cut and paste the above URL if the line is broken) or send a blank email to leave-16294-13238.0e762b65028402721e10bbc97ede5...@fsulist.frostburg.edu<mailto:leave-16294-13238.0e762b65028402721e10bbc97ede5...@fsulist.frostburg.edu> -------------------------------------------------- Hugh J. Foley Department of Psychology Skidmore College Saratoga Springs, NY 12866 518-580-5308 http://www.skidmore.edu/~hfoley -------------------------------------------------- "And I still don't know if I'm a falcon, a storm, or an unfinished song." Rilke -------------------------------------------------- --- You are currently subscribed to tips as: [email protected]. To unsubscribe click here: http://fsulist.frostburg.edu/u?id=13090.68da6e6e5325aa33287ff385b70df5d5&n=T&l=tips&o=16296 or send a blank email to leave-16296-13090.68da6e6e5325aa33287ff385b70df...@fsulist.frostburg.edu
