拟泊松似然估计（PPML）。
我使用的是STATA 11.在回归时显示：
ppml chinaexports lngdp lndistcircle

this is version 11.0 of Stata; it cannot run version 11.1 programs
     A free update to version 11.1 is available; type update and follow the
     instructions, or visit http://www.stata.com/support/updates.  If you are not
     connected to the Internet, you can contact StataCorp to obtain the latest version.
r(9);