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From |
"Nick Cox" <n.j.cox@durham.ac.uk> |

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

Subject |
RE: st: about dummy variables |

Date |
Thu, 17 Mar 2005 17:31:41 -0000 |

I agree largely with Daniel here, although a bigger difficulty is that I don't understand your specific questions. Nevertheless given your code note that you could define your dummies much more concisely: First, if year==1986 | year==1987 | year==1988 | year==1989 | year==1990 | year==1991 | year==1992 | year==1993 | year==1994 | year==1995 collapses to if inrange(year, 1986, 1995) so that the code then looks like gen pre96=0 replace pre96= 1 if inrange(year, 1986, 1995) Second, that in turn collapses to gen pre96 = inrange(year, 1986, 1995) Similarly gen pos86 = inrange(year, 1996, 2002) However, it would seem that you wouldn't normally need both these dummies. Nick n.j.cox@durham.ac.uk Tae Hun Kim > Yes, I know this question is not directly related to State program. > But I need someone's help to understand the problem. > > The Ststa code to generate dummy variables is > **dummy variable > gen pre96=0; > replace pre96=1 if year==1986 | year==1987 | year==1988 | year==1989 | > year==1990 | year==1991 | year==1992 | year==1993 | year==1994 | > year==1995; > gen post96=0; > replace pos96=1 if year==1996 | year==1997 | year==1998 | year==1999 | > year==2000 | year==2001 | year==2002; > > **interactive term > gen pre96s=pre96*smom; > gen post96s=post96*smom; Daniel Lawson > > This doesn't sound like a Stata-specific question. You > might get more > > or better responses to this question if you asked this question in > > another forum. > > > > In terms of Stata code, how did you generate your dummy variable? Tae Hun Kim > > > I would be greatfull if someone helps me to understand > dummy variables. > > > > > > For example, > > > Y= a+b*X+c*dum+d*(X*dum)+e > > > where Y : income, X : education(years), dum=dummy > variable (dum=1 if > > > male, dum=0 if female) > > > > > > If I estimate this equation using OLS, recalulated > coefficents are the > > > same regardless of whether dum=1 if male (0 otherwise) or > dum=1 if > > > female(0 otherwise). > > > i.e. if I define a dummy variable as dum=1 if male, o otherwise > > > male(1) : E(Y)= (a+c)+(b+d)X, female(1) : E(Y)=a+bX > > > if I define a dummy variable as dum=1 if female, o otherwise > > > male(2) : E(Y)=a+bX, female(2) : E(Y)= > (a+c)+(b+d)X > > > =>the coefficients of male(1) and male(2) equation are the same. > > > > > > (Question) > > > 1. But when I estimate this equation using GMM or IV, the > coefficients > > > of male(1) and male(2) equation are not the same. Why? > > > 2. If the coefficient are different, what is the criteria > to define a > > > dummy variable(male=1 or female=1) ? > > > * * 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/

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