A quick mathematics refresh: level, difference, and rate

A level tells where the series is. A first difference tells how much it changed. Percent change puts that change relative to the previous level. Scrub through the series and locate its largest increase.

Indexed values. To compare two series that start at different levels, or in different units, rebase each to 100 at the same base period: \( I_t = 100 \cdot y_t / y_b \), where \( y_b \) is the value in the base period. A campus that grows from 400 to 440 and one that grows from 50 to 55 both read 110, because each is 10% above its baseline. An indexed chart, with every series at 100 in the base period, therefore compares relative (percent) change from that baseline; it no longer shows the original sizes, and a different base period can change the picture.