用极大似然方法估计空间面板数据，当模型设定为spmlb<spml(fm,data=b,index=c("DQ","NF"),listw=gal.mat,effect="individual",model="within",lag=T,spatial.error="b")时可以估计出结果，即个体固定效应！当模型设定为spml2b<spml(fm,data=b,index=c("DQ","NF"),listw=gal.mat,effect="individual",model="random",lag=T,spatial.error="b")时，怎么这样报错Error in if (!balanced) stop("Estimation method unavailable for unbalanced panels") :   missing value where TRUE/FALSE needed！这是为什么？谢谢了