The EV is used to calculate the worth of any casino bonus. The formula contains three variables: wagering requirement, maximum cashout, and game house edge.
If your free spins bonus is applicable on a slot machine with a 2.5 percent house edge, ,$100 max cashout, and 20X wagering requirement (20 x $100) = $2000:
EV = (maximum cashout) – (wagering requirement) x (house edge)
EV = (100) – ($2000) x (2.5%)
EV = 100 – $50
EV = $50
Since the bonus is the example above has a +ve EV, it is definitely worth redeeming. Had it been -$50, it is not valuable for redeeming.