用hlm做模型练习的时候，<br>
结果总是出现HLM2 is unable to compute starting values based on the specified model.<br>
Every level-1 predictor matrix is near singular.<br>
1) Center one or more level-1 predictors.<br>
2) One (or more) of the random effects should be either deleted from the model<br>
   or treated as fixed.<br>
将第二层的随机效果全部取消，可是只有sigma_square，没有tau，数据和论文的结果不一样，这个是怎么回事