如题，鄙人使用的命令为
  proc panel data=cars;
  model cabret=deratio lnasset roe/fixtwo vcomp=fb;model cabvolat=deratio lnasset roe/fixtwo vcomp=fb;
model cabiliq=deratio lnasset roe/fixtwo vcomp=fb;model cabtrdamt=deratio lnasset roe/fixtwo vcomp=fb;
model cabturnrate=deratio lnasset roe/fixtwo vcomp=fb;
  id stkcd evt;run;quit;
但是结果却是
There is only one cross section or time series observation. Computations will be terminated.
  求问这是怎么回事呢？
  谢谢