To estimate the number of years it takes a variable to double we can use the rule of 70 or the rule of 72. It is best to use the rule of 72 if: To estimate the number of years it takes a variable to double we can use the rule of 70 or the rule of 72. It is best to use the rule of 72 if:

a. the annual growth rate (percentage change) is 5% or larger.

b. the annual growth rate is less than 5%.

c. the annual growth rate is larger than 10%.

d. the annual growth rate is less than 4%.