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03-24-2015 04:37 PM

Hello,

I perform a multinomial logistic regression using proc logistic and glogit, which results with those parameters:

Table1 | Cat1 | Cat2 | Cat3 | Car4 | Cat6 | Cat7 | Cat8 |

Intercept | -3.09960 | -2.27930 | -5.15200 | -4.33730 | -0.30360 | -1.70810 | -0.87280 |

Age65_69 (ref) | |||||||

Age70-74 | -0.15780 | -0.13040 | -0.40460 | -0.48680 | 0.10860 | -0.04320 | 0.31830 |

Age75-79 | -0.82400 | -0.62110 | -0.39450 | -0.53650 | 0.23330 | -0.29800 | 0.82200 |

Age80_84 | -0.60620 | -0.78130 | -0.65530 | -0.60930 | 0.28750 | -0.25480 | 1.29840 |

Age85 | -0.79490 | -0.04920 | -0.04200 | 0.12350 | 0.27580 | -0.23910 | 1.33200 |

The odds ratios are thus:

Table2 | Cat1 | Cat2 | Cat3 | Car4 | Cat6 | Cat7 | Cat8 |

Age70-74 | 0.85402 | 0.87774 | 0.66724 | 0.61459 | 1.11472 | 0.95772 | 1.37479 |

Age75-79 | 0.43867 | 0.53735 | 0.67402 | 0.58479 | 1.26276 | 0.74230 | 2.27505 |

Age80_84 | 0.54542 | 0.45781 | 0.51929 | 0.54373 | 1.33309 | 0.77507 | 3.66343 |

Age85 | 0.45163 | 0.95199 | 0.95887 | 1.13145 | 1.31758 | 0.78734 | 3.78861 |

There are more independent variables, but for my problem, I don’t think they are relevant.

When I convert parameters to get the predicted probabilities, I have this:

Table3 | Cat1 | Cat2 | Cat3 | Car4 | Cat6 | Cat7 | Cat8 |

Age65_69 (ref) | 0.01800 | 0.04164 | 0.00246 | 0.00556 | 0.31584 | 0.11333 | 0.29467 |

Age70-74 | 0.01420 | 0.03362 | 0.00150 | 0.00312 | 0.32008 | 0.09929 | 0.36482 |

Age75-79 | 0.00637 | 0.01784 | 0.00129 | 0.00253 | 0.30894 | 0.06451 | 0.48730 |

Age80_84 | 0.00658 | 0.01262 | 0.00082 | 0.00194 | 0.26923 | 0.05258 | 0.60482 |

Age85 | 0.00531 | 0.02554 | 0.00149 | 0.00398 | 0.26300 | 0.05235 | 0.61282 |

My problem is the following: when I take those probabilities and try to calculate the odds ratios, the results are not the same than those computed by SAS. For example, the odds ratio for Age60-74, cat1: (0.0142/(1-0.0142))/(0.018/(1-0.018))=0.7856, while the one indicated in table 2 is 0.85402.

I guess I made a mistake somewhere, but I don’t know where. Or maybe it’s just something I don’t understand yet. Moreover, I’m not totally sure the the proc logistic with glogit is the right method to perform a multinomial logistic regression. I’m on that problem since 3 days, so if you have the answer, it would be much appreciated.

Thanks