[U] User's Guide

[ U ] 11.4 varlists 107You can select a range of levels—a range of virtual variables—by using the i(numlist).varname.This can be useful when specifying the model to be fit using estimation commands. You may notomit the i when specifying a numlist.examplesdescriptioni2.cat single indicator for cat = 22.catsame as i2.cati(2 3 4).cat three indicators, cat = 2, cat = 3, and cat = 4; same asi2.cat i3.cat i4.cati(2/4).cat2.cat#1.sexi2.cat#i1.sexsame as i(2 3 4).cata single indicator that is 1 when cat = 2 and sex = 1 and is 0 otherwisesame as 2.cat#1.sex11.4.3.5 Applying operators to a group of variablesFactor-variable operators may be applied to groups of variables by using parentheses. You maytype, for instance,i.(group sex arm)to mean i.group i.sex i.arm.In the examples that follow, variables group, sex, arm, and cat are categorical, and variablesage, wt, and bp are continuous:examplesi.(group sex arm)group#(sex arm cat)group##(sex arm cat)group#(c.age c.wt c.bp)group#c.(age wt bp)expansioni.group i.sex i.armgroup#sex group#arm group#cati.group i.sex i.arm i.cat group#sex group#armgroup#catgroup#c.age group#c.wt group#c.bpsame as group#(c.age c.wt c.bp)Parentheses can shorten what you type and make it more readable. For instance,. regress y i.sex i.group sex#group age sex#c.age c.age#c.age sex#c.age#c.ageis easier to understand when written as. regress y sex##(group c.age c.age#c.age)