请教各位大大，我的数据是panel data，36个国家，1990-2009,5-year period，因变量是growth rate of real GDP per capita,自变量有log of initial real GDP per capita, log of initial secondary school enrollment rate, government expenditure, foreign direct investmente和M2，回归方法用的是TSLS，目前工具变量选择了initial value of m2, initial value of foreign direct investment. initial value of government expenditure能不能作为工具变量呢，还有别的没？起码需要5个工具变量，希望各位高手能给点意见，指导下，谢谢