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From |
khigbee@stata.com |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: Re: st: CI for adjusted mean |

Date |
Wed, 24 Oct 2007 09:18:54 -0500 |

Maarten buis <maartenbuis@yahoo.co.uk> asks: > ... I expected that this code: > > sysuse auto, clear > recode rep78 1/2 = 3 > anova mpg rep78 wei, continuous(wei) > adjust wei, se ci > > would give me a mean of mpg adjusted for wei (with standard error and > confidence interval), but instead it gives me an empty table. Does > anyone have any thoughts on this one? The description paragraph for -adjust- (manual or help file) says: ... If by() is not specified, adjust produces results as if by() defined one group. Variables used in the estimation command but not included in either the by() variable list or the adjust variable list are left at their current values, observation by observation. Here adjust displays the average estimated prediction (or the corresponding probability or exponentiated prediction), substituting the mean of these unspecified variables within each group defined by the variables in the by() option. Since -by()- was not specified, we are dealing with one group (the whole estimation sample) and since rep78 was not specified in the -adjust- command, the mean of rep78 over the whole estimation sample is used in the prediction (see the last part of the description "... substituting the mean ..."). With your example the mean of rep78 over the estimation sample is 3.57971. After an -anova- where rep78 is in the model as a categorical variable, -predict- (and hence -adjust- which uses -predict-) returns a missing value when asked for the prediction of the model where the categorical variable is set to a value that was not one of the levels of that categorical variable. That is why you end up with an empty table--the -predict- command returned a missing value for the prediction. You could instead do things like . adjust wei rep=3, se ci . adjust wei rep=4, se ci . adjust wei rep=5, se ci or more easily . adjust wei, by(rep) se ci With these cases, the rep78 variable is treated as a categorical variable (like it should be in this -anova- example). Ken Higbee khigbee@stata.com StataCorp 1-800-STATAPC * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: CI for adjusted mean***From:*Rachele Capocchi <rachele.capocchi@arsanita.toscana.it>

**Re: Re: st: CI for adjusted mean***From:*Maarten buis <maartenbuis@yahoo.co.uk>

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