Graphic: know the score in Battle of the Mersey

Are you sitting comfortably? Have you got a pen? I am going to teach you how to do your own Fink Tank.

Well, not quite. I am going to help you to make your own rough-and-ready estimation of match-day probability. With just a few figures and a calculator, you should be able to do it in the pub on the way to the match.

Dr Henry Stott, Dr Ian Graham and Dr Mark Latham have been at work stripping down our model to give you something workable. It isn’t, obviously, as sophisticated as the Fink Tank Predictor, but then what is?

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I am going to use tomorrow’s big FA Cup fourth-round tie — Liverpool v Everton — as an example.

First, you need to understand home advantage. A study of 3,257 games suggests that the average team away from home against average opponents score 1.1 goals, while at home they score 1.49 goals. This produces a critical number for the DIY Fink Tank. If you divide 1.49 by 1.1, you get 1.36. For every goal you think your team would score away from home, they would score 1.36 at home.

Right. Next step. And for this you just need the league table. You are going to calculate how many goals your team score and concede per game at home and away, and how many your opponents score and concede per game. This allows us to get ratings for both sides in attack and defence.

Let’s say you are a Liverpool fan. At home in 2007-08 and 2008-09 the team have scored 60 goals in 30 matches — an average of two per game. We want comparable ratings for both sides, so for the moment we are going to pretend that both sides are away from home. So divide that two goals per game by 1.36. You get 1.47. Jot it down.

Now look at Liverpool’s away goals. They have scored an average of 1.43 goals. Taken together with the home average of 1.47 goals that means Liverpool score an average of 1.45 per game. That is their attack rating. You can do the same for Everton. Their attack rating is 1.21.

Repeat the exercise for defence. The only thing you need to remember here is what to do about home advantage. Don’t forget that when Liverpool concede goals away from home it is the other team who had the home advantage. So the goals scored against Liverpool when they are away need to be divided by 1.36. For defence, then, we get a rating of 0.6 for Liverpool and 0.86 for Everton.

What now? For Liverpool multiply 1.36 by Liverpool’s attack rating (1.45) and Everton’s defence rating (0.86). You get an expected number of Liverpool goals of 1.7. For Everton multiply their attack rating (1.21) by Liverpool’s defence (0.6). You get 0.73. So 1.7 plays 0.73, an average scoring game with Liverpool in the driving seat, essentially expected to win by a goal.

If you had a computer and a spreadsheet you could now use something called the Poisson distribution to get the probability of a Liverpool victory. The graphic shows how probable different scores are and it follows this common distribution.

But you are in the pub, so maybe it is difficult to work an Excel sheet and perhaps you don’t know what the Poisson distribution is anyway.

Here is the quick cheat method. Liverpool’s expected goal difference is 0.97. Multiply this by 21 and then add 38.5. You can do this (expected goal difference times 21 plus 38.5) for almost any side to get the probability of victory. It gives you a 59 per cent chance of a Liverpool victory. Not a bad estimation at all.

There you have it. The Fink Tank on the back of an envelope.