To measure relative change over time in a variable, Multiple choice question. 

an index is calculated by choosing a base period and multiplying the base value by the current value. 

an index is calculated by choosing a base period and taking a ratio of the current period to the base period. 

an index is calculated by taking the ratio of the the previous period to the current period for each new period. 

an index is calculated by subtracting a base period from the current period