GRE Data Interpretation Question

[Arun315]

Hi 

Can anyone help to clarify in understanding the below graph. It is from one of the model test in barron's gre book. My doubt is what does Index  mean in the graph. 

 

 

Edited July 6, 2016 by Arun315

[Brent@GreenlightGRE]

The chart's creator decided to make 1980 the benchmark year (with an index of 100). 

So, for other years, the index tells us the enrollment for a certain year as a percentage of the enrollment in 1980

For example, if the index for a given year is 90, then the enrollment for that particular year = 90% of the  enrollment in 1980

Cheers,

Brent