请问我用R语言geeglm包做广义估方程分析4次随访肿瘤纵横比差别得到有意义结果，想用glht做两两比较，总报错。代码为：>fit2=geeglm(纵横比~time,data=b,id=id,family=gaussian,corstr="exchangeable") > b$id <- as.factor(b$id)

> b$time <- as.factor(b$time)

> b$纵横比 <- as.factor(b$纵横比)

> summary(glht(fit2,mcp("time" = "Tukey")))

Error in mcp2matrix(model, linfct = linfct) : 

 Variable(s) ‘time’ of class ‘integer’ is/are not contained as a factor in ‘model’.