1. 建立新变量

gen z=x*y

2. addition to typing variable names from your data, you can type factor variables, which might look like

i.varname
i.varname#i.varname
i.varname#i.varname#i.varname
i.varname##i.varname
i.varname##i.varname##i.varname

  Operator  Description

i. unary operator to specify indicators
c. unary operator to treat as continuous
# binary operator to specify interactions
## binary operator to specify factorial interactions

3. Here are some examples of use of the operators:

 Factor            
         specification     Result
         -------------------------------------------------------------------------------------------------------------
         i.group           indicators for levels of group

         i.group#i.sex     indicators for each combination of levels of group and sex, a two-way interaction

         group#sex         same as i.group#i.sex

         group#sex#arm     indicators for each combination of levels of group, sex, and arm, a three-way interaction

         group##sex        same as i.group i.sex group#sex

         group##sex##arm   same as i.group i.sex i.arm group#sex group#arm sex#arm group#sex#arm

         sex#c.age         two variables -- age for males and 0 elsewhere, and age for females and 0 elsewhere; if age
                             is also in the model, one of the two virtual variables will be treated as a base

         sex##c.age        same as i.sex age sex#c.age

         c.age             same as age

         c.age#c.age       age squared

         c.age#c.age#c.age age cubed
         -------------------------------------------------------------------------------------------------------------


Base levels

    You can specify the base level of a factor variable by using the ib. operator.  The syntax is

           Base         
           operator(*)    Description
           -----------------------------------------------------------------------------------------------------------
           ib#.           use # as base, #=value of variable
           ib(##).        use the #th ordered value as base (**)
           ib(first).     use smallest value as base (the default)
           ib(last).      use largest value as base
           ib(freq).      use most frequent value as base
           ibn.           no base level
           -----------------------------------------------------------------------------------------------------------
            (*) The i may be omitted.  For instance, you may type ib2.group or b2.group.
           (**) For example, ib(#2). means to use the second value as the base.

    If you want to use group==3 as the base in a regression, you can type,

        . regress y  i.sex ib3.group

    You can also permanently set the base levels of categorical variables by using the fvset command.