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From |
"Joseph Coveney" <jcoveney@bigplanet.com> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
st: Re: Test for homogeneity of odds ratios with ordinal data (r x 2)? |

Date |
Wed, 20 May 2009 22:20:34 +0900 |

Mike Schmitt wrote: I'm somewhat familiar with the Mantel-Haenszel test for homogeneity of odds ratios for a 2 x 2 structure, however I'm currently working on a project that is part of a randomized control trial, although the outcome I'm looking at was not specifically targeted by the intervention. I want to ensure/check that the intervention did not have an unexpected effect. I have intervention(=1) and control(=0) groups, an ordinal outcome measure(0 through 4), and several dichotomous predictor variables. For a 2 x 2, I could use: "mhodds dep_var predict_var, by(intervention)" However, I'm running into a wall with the ordinal dependent/outcome measure. I've found a few papers that speak of extending the principles behind the MH test and another method using a proportional odds model, however I do not possess the skill set to understand how to implement this in Stata. Does anyone have any pointers or possible avenues for exploring this issue? Thanks, -------------------------------------------------------------------------------- I'm not familiar with the literature that you're alluding to, but fitting a proportional odds model is straightforward in Stata using the official command, -ologit-. Inasmuch as Mantel-Haenszel testing of homogeneity of odds ratios is equivalent to testing the predictor-by-stratification variable interaction term in logistic regression, I would expect that you could do an analogous test with -ologit-, e.g., generate byte iXp = intervention * predictor ologit outcome intervention predictor iXp, or test iXp This assumes that the dichotomous predictor variable is coded 0/1 just as you show for the intervention variable. You can also perform a likelihood ratio test via the official -lrtest-, and there is a user-written command, -testomit-, that allows a score test. Plotting the data might be more beneficial than testing. There is user-written command, -distplot-, that comes in handy for ordered-categorical response data. Syntax for it in your circumstances would be something like: distplot outcome, over(intervention) by(predictor) [other options, such as -midpoint-] With these plots, you're often looking for rightward or leftward shifts of the curves (the curves roughly parallel). For that, it's sometimes helpful to put in a visual reference line using the option -yline(0.5)-. You're concerned about heterogeneous odds ratios, and so I suggest looking whether there are dramatic differences in the spread between control and experimental treatment group curves between predictor groups, or even transposition of the treatment group curves between predictor group categories. Comparison of slopes and shapes of the lines can give insight into model assumptions. Joseph Coveney * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: Test for homogeneity of odds ratios with ordinal data (r x 2)?***From:*Mike Schmitt <mrschmitt@gmail.com>

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