我在做clogit的过程中遇到一个问题,

无约束模型：clogit  y x1 x2 x3 x4,group(id)

那么该如何做约束模型(constraint model)

我的尝试

constraint define 1 x1=0

constraint define 2 x2=0

constraint define 3 x3=0

constraint define 4 x4=0

clogit y x1 x2 x3 x4, group(id) constraint(1-4)

结果显示

note: multiple positive outcomes within groups encountered.
note: 1 group (21 obs) dropped due to all positive or
 all negative outcomes.
there are at least as many constraints as parameters

不怎么理解上面的意思，请帮忙看一下。3x