You will often see the term EV used on Profit Accumulator, this stands for Expected Value or Estimated Value. This isn't the value we expect to get every time, but instead the value we expect to get on average if we did an offer an infinite amount of times.
If an offer has a positive expected value (+EV), we have the edge and it is worth doing. If it has a negative expected value (-EV), then you will lose money in the long run. The larger the EV, the more money you're likely to make. Profit Accumulator will always work out the value of the offers for you, but it's nice to know where the numbers are coming from.
Stripped down to its basic and general form, the value of something is the chance of an event happening, multiplied by your profit/loss if that event does happen, then total all the possible scenarios.
EV = [(Probability event 1) x (Profit/Loss if event 1 happens)] + [(Probability event 2) x (Profit/Loss if event 2 happens)] + [(Probability event 3) x (Profit/Loss if event 3 happens)] + etc.
Tip: The probability of an outcome happening in sports can be best gauged by doing 1 divided by your lay odds then multiply by 100. Eg. If the lay odds on Arsenal are 1.25, the chance of them winning are 1 / 1.25 = 0.8, then 0.8 x 100 = 80% chance.
Now back to working out the expected value. This might seem complicated, but it's actually pretty simple. Here's an example using a coin toss game:
Game – A coin is tossed. If the result is heads, we win £10, if the result is tails we lose £5.
Probability of heads = 50% or 0.5, probability of tails = 50% or 0.5.
EV = (0.5 x £10) + (0.5 x -£5) = £5 - £2.50 = £2.50 – Since this is positive we have a +EV game. If we played this game an infinite amount of times we would expect to make on average £2.50 each time we play it. We wouldn't have any way of laying this game to guarantee profit, but we should still be very keen to play it.
There's more on expected value and variance in the casino strategy guide.