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From |
Maarten buis <maartenbuis@yahoo.co.uk> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Areg, absorb |

Date |
Mon, 11 Apr 2011 14:03:16 +0100 (BST) |

--- On Mon, 11/4/11, emanuele mazzini wrote: > do you know a way to not omit the variables that the > command xi i.varname generates? I tried with the option > noomit, but it seems that it does not work, i.e. it > still keeps on omitting the first country of my sample. Imagine you have two countries Aistan and Bland and that we want to predict a variable y. Lets first understand what happens when we omit one of the dummies. In this case assume we use one dummy variable called bland, which is 1 when the country is Bland and 0 when it is not Bland (and thus Aistan). In that case we ommited the dummy aistan. In this case we have the following equation: y_hat = b0 + b1 * bland If the country is Bland than its predicted values is y_hat = b0 + b1 * 1 = b0 + b1 If the country is Aistan than its predicted value is y_hat = b0 + b1 * 0 = b0 So the constant is the predicted y for Aistan and b1 is the difference in predicted y between Aistan and Bland. What will happen when we also include the dummy aistan? In this case we have the following equation: y_hat = b0 + b1 * bland + b2 * aistan If the country is Bland than its predicted values is y_hat = b0 + b1 * 1 + b2 * 0 = b0 + b1 If the country is Aistan than its predicted value is y_hat = b0 + b1 * 0 + b2 * 1 = b0 + b2 So now there are three parameters to represent two predicted values, which means that one of these is unidentified. For example we could think that b0 is 2, than b1 is the predicted y - 2 for Bland and b2 is the predicted y - 2 for Aistan. Or we could think that b0 is 3, than b1 is the predicted y - 3 for Bland and b2 is the predicted y - 3 for Aistan. You can see that you can get exactly the same predictions for different values of b0, just by adjusting the two remaining parameters. There is thus no way to distinguish the fit of these different models. In order to be able to estimate the model you must constrain one of the parameters. Be default we constrain the parameter of one of the dummies to be 0 (i.e. we just exclude that variable from our model). Alternatively we could constrain the constant to be 0, with the -nocons- option. Anyhow, from your previous question I gathered that you are not interested in these effects, you even want to suppress the display of these variables. In that case I would just stick to the default, all these models are mathematically equivalent anyhow. But if you are substantively interested in the effects of these variables, than this can sometimes be a really nice trick that can help the interpretation of your model. Notice however, that this does not change your model, just the way it is displayed. Hope this helps, Maarten -------------------------- Maarten L. Buis Institut fuer Soziologie Universitaet Tuebingen Wilhelmstrasse 36 72074 Tuebingen Germany http://www.maartenbuis.nl -------------------------- * * 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/

**Follow-Ups**:**Re: st: Areg, absorb***From:*emanuele mazzini <madsoenistata@gmail.com>

**References**:**Re: st: Areg, absorb***From:*emanuele mazzini <madsoenistata@gmail.com>

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