题目给了两组数据，spi(stock price index) and cpi(consumer price index), 现在叫estimate the relationship between the stock price inflation and the consumer price inflation, 规定spi 和cpi都要取自然对数。问题是我应该在取对数之前就定义inflation=(cpi-cpi(-1))/cpi(-1)*100,stock return=(spi-spi(-1))/spi(-1) *100,再统一取对数，还是先对spi and cpi取对数后，再定义inflation=lncpi-lncpi(-1)  and stock return=lnspi-lnspi(-1)呢？两种方法得出来的结果不同，后一种不能证明fisher equation.