AI articles : 1000

AI appearances : 3500

Unemployment articles : 200

Unemployment appearances : 300

Intersection articles : 50

Intersection AI appearances : 100

Index =
Intersection AI appearances / Unemployment appearances
=
100 / 300
= 0.333


i.e. no. of appearances of "AI" in articles containing "unemployment"
divided by no. of appearances of "unemployment" across all articles