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From |
Urmi Bhattacharya <ub3@indiana.edu> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
st: Query about finding predicted change in probability after logit for changing two variables |

Date |
Thu, 9 Jun 2011 10:47:20 -0400 |

Dear Statalisters, I am running a logit with multiple factor variables as well as continuous variables. I am interested in finding the estimated predicted change in the probability with changing two dummy variables along with the standard errors and the p values. This was done by Thomas DeLeire (The wage and Employment effects of The Americans with disabilities act), Journal Of Human Resources, 2001. I am explaining it using an example. sysuse auto, clear drop if rep78==. tab rep78 /***generating dummies for category of rep78****/ generate cqual_low=1 replace cqual_low=0 if rep78 >2 generate cqual_medium=1 replace cqual_medium=0 if (rep78 == 1) | (rep78 == 2)| (rep78 == 5) generate cqual_high=1 replace cqual_high=0 if (rep78 == 1) | (rep78 == 2)| (rep78 == 3)| (rep78 == 4) /***generating dummies for high turn and low turn*/ generate turn_low=1 replace turn_low=0 if turn >35 generate turn_high=0 replace turn_high=1 if turn >35 /******i run the logit****/ logit foreign i.cqual_high i.cqual_medium i.turn_high headroom mpg trunk /*******now i want to find what is the predicted change in the probability of foreign for those cars with cqual_high=1 with turn_high=1 from those with cqual_medium=1 and turn_low=1 */ generate xb1= _b[_cons ] +_b[1.cqual_high ] +_b[1.turn_high ]+_b[headroom ]*headroom +_b[mpg]*mpg +_b[trunk]*trunk generate xb2= _b[_cons ] +_b[1.cqual_medium] +_b[headroom ]*headroom +_b[mpg]*mpg +_b[trunk]*trunk predictnl pred_change = (1+exp((-1)*xb1))^-1 - (1+exp((-1)*xb2))^-1 Doing this I get the predicted change for each observation. However, the paper just presents one estimate and the standard error for the predicted changes in probabilities. 1.Does this imply that I have to consider the predicted changes at means of other variables? In that case, how do I get that with the standard errors and p values? 2.Or does this mean, I have to ignore all other variables while calculating the predicted probabilities? This however makes little sense to me. 3. Do I take an average of the predicted changes and then find the standard errors and p values of it. Going through the paper, I could not figure out which of the above three was used. Any help would be greatly appreciated. Best Urmi Bhattacharya * * 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: Query about finding predicted change in probability after logit for changing two variables***From:*Austin Nichols <austinnichols@gmail.com>

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