问题如下：
The following presents selected commodity price data for July–August 2015:
Given the consumption basket and prices presented, which type of price index will result in the highest calculated inflation rate over a two-month time period?
选项：
A.One that uses a current consumption basket
B.One that uses a constant consumption basket
C.One reflecting substitutions made by consumers over time
解释：
B is correct.
The inflation rate calculated by using a constant consumption basket (the Laspeyres index) is 10%, derived as follows:
July 2015 consumption basket = (18 × €1) + (6 × €2) = €30
August 2015 consumption basket = (18 × €1) + (6 × €2.5) = €33
Value of the Laspeyres index (IL) = (€33/€30) × 100 = €110 Inflation rate = (110/100) – 1 = 0.10 = 10%
The inflation rate calculated using a current consumption basket (the Paasche index) is 8%, derived as follows:
July 2015 consumption basket = (17 × €1) + (4 × €2) = €25
August 2015 consumption basket = (17 × €1) + (4 × €2.5) = €27
Value of the Paasche index (IP) = (€27/€25) × 100 = €108
Inflation rate = (108/100) – 1 = 0.08 = 8%
The inflation rate calculated by "chaining" the monthly prices of consumption baskets as they change over time (the Fisher index) is derived as follows: Value of the Fisher index = (Ip*IL)^0.5
Value of the Fisher Index =(110*108)=108.99
Inflation rate = (108.99/100) – 1 = 0.0899 = 8.99%
解析：在拉氏指数法下，应该是用基期的消费篮子:
7月物价水平：(18 × € 1) + (6 × € 2) = € 30
8月物价水平：(18 × € 1) + (6 × € 2.5) = € 33
通胀指数：Inflation rate=(33/30)-1=10%
在沠许指数法下,应该是用当期的消费篮子：
7月的物价水平：(17 × € 1) + (4 × € 2) = € 25
8月的物价水平：(17 × € 1) + (4 × € 2.5) = € 27
通胀指数：(27/25)-1=8%
费雪指数：(110*108)^0.5= € 108.99
通胀指数：(108.99/100) – 1 = 0.0899 = 8.99%
由此可见B选项正确。
请问老师，不是很懂为什么通胀指数直接就是：(27/25)-1=8%，为什么要-1呢，这个带“-1”的算法是对应哪个公式的呢，如果按步骤来是不是应该是先（27/25）*100即算出八月的指数吗然后再算七月的然后才八月-七月的/七月的算出来八月的沠许指数，这样的算出来不才是沠许指数法下的吗，还有七月的指数怎么算呢，算八月的时候七月是基期那么算七月的时候呢几月是基期呢，谢谢
