回归模型中的变量设置没有发生变化，但多个时期下变量的数据发生变化。这种情况下两个模型间相同变量的系数对比没有意义吗？
附论文中看到的原句：
Because population and GDP values changed over time, it is difficult to associate variations in their estimated coefficients to changes of their impacts on urban expansion over different periods.