tree structure specified for the nested logit model

type  N      severity N   k
------------------------------
low   99 --- 1        99  30
high 198 --- 2        99  55
           +- 3        99  14
------------------------------
               total  297  99

k = number of times alternative is chosen
N = number of observations at each level

. nlogit choice  Cause Weather||type: AccidentForm  ControlType RoadType||severity:,case( accident
> _id)
variable Cause is not alternative-specific: it has no within-case variability
r(459);


其中cause是事故原因代码，是分类数据，求问红字原因，新手初学