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Stata Technical Bulletin

Controlling the omitted dummy

By default, i .aamame omits the dummy corresponding to the smallest value of varname; in the case of a string variable,
this is interpreted as dropping the first in an alphabetical, case-sensitive sort. xi provides two alternatives to dropping the first:
xi will drop the dummy corresponding to the most prevalent value of varname or xi will let you choose the particular dummy
to be dropped.

To change xi’s behavior to dropping the most prevalent, you type,

. global S-XIMODE "prevalent"

although whether you type “prevalent” inside the quotes or “yes” or anything else does not matter. You need type this command
only once per session and, once typed, it affects the expansion of all categorical variables. If, during a session, you want to
change the behavior back to the default drop-the-first rule, you type

. global S.XIM0DE

Once you set S-XIMODE, i. varname omits the dummy corresponding to the most prevalent value of varname. Thus, the coefficients
on the dummies have the interpretation of change from the most prevalent group. For example,

. global S-XIMODE "prevalent"

. xi: regress y i.agegrp

might create Iagegr_l through Iagegr.4 and would result in Iagegr_2 being omitted if agegr = 2 is most common (as
opposed to the default dropping of Iagegr-I). The model is then:

y = bo + bι Iagegr_l + b$ I agegr_3 + Ô4 I agegr _4 + и

Then,

Predicted у for agegrp 1 = bo + δ₁ Predicted у for agegrp 3 = ô₍j l ô₃

Predicted y for agegrp 2 = bo         Predicted y for agegrp 4 = 0_υ + 0∣

Thus, the model’s reported o or z statistics are for a test of whether each group is different from the most prevalent group.

Perhaps you wish to omit the dummy for agegrp 3 instead. Whether you have set the global macro S-XIMODE or not, you
do this by creating a global macro with the same name of the variable containing “xi о mit value ”. In this case:

. global agegrp ^,,xi omit 3”

Now when you type

. xi: regress y i.agegrp

I agegr_3 will be omitted and you will estimate the model:

y = b'_t + b'₁ Iagegr-I + b'₂ I agegr_2 + b'₄ I agegr_4 + и

Later, if you want to return to the default omission, you type

. global agegrp

thus clearing the macro.

In summary, i .vammme omits the first group by default but if you define

. global S-XIMODE "prevalent"

then the default behavior changes to that of dropping the most prevalent group. Either way, if you define a macro of the form

. global varname ^,,xi omit #’•

of, if varname is a string,

. global varname "xi omit string-literal"

then the specified value will be omitted.

Examples:    . global agegrp "xi omit 3"

. global race "xi omit White" (for race a string variable)

. global agegrp                     (to restore default for agegrp)

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