Calculate the confidence interval of the index of a particular year by adding as well as subtracting twice, or being more precisely 1.96, its standard error. If the interval covers 1, the index is not different from the base year. Else, there is a significant difference. Example: the index of year 5 = 0.91 with a standard error of 0.02. The confidence interval then ranges from 0.87 and 0.95, and the index is significantly lower than the base year. In this way, all indices can be tested against the base year, but testing against other years is not directly possible. You have to select another base year first.