WANNABE Blue Adam Saad has emerged as the most valuable footballer up for grabs in this year's AFL Trade Period.

An advanced player valuation metric best known in baseball, Wins Above Replacement (WAR), rates the tearaway Bomber as having the best season among those mooted as trade targets.

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The two men behind the AFL's answer to WAR quantify it as how many wins a player contributes to his team compared to a replacement-level athlete at the same position.

The replacement level is based on the average output of a footballer selected in a rookie draft – players overlooked by every club several times – who played games between 2012 and 2019.

It's not a foreign metric in AFL circles, with baseball scouts Brett Ward and Jon Deeble, on behalf of their company Aphex, presenting a similar concept about seven years ago.

Clubs still use Aphex software to help make list management decisions.

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Analytics specialists Christos Manoussakis and Charles Roberts, who play for Old Trinity in the VAFA, are the brains behind the WAR model that highlights why Carlton is so keen to secure Saad.

Demon Christian Petracca had the highest WAR this year – almost two-and-half wins more than a replacement-level midfielder – ahead of seven other 2020 All-Australians in the top 10.

The higher the WAR value the better, while a minus-WAR means those players effectively cost their side wins.

2020 'WINS ABOVE REPLACEMENT' RANKINGS

Saad came in at 1.263, more than double what he recorded in his first two seasons at Essendon, after crossing from Gold Coast.

Even then, his WAR scores in 2019 (0.537) and 2018 (0.547) were superior to what Jordan De Goey (0.483), Shaun Higgins (0.440), Jeremy Cameron (0.430) and Jared Polec (0.425) produced this year.

The likes of Brad Crouch, Orazio Fantasia, Ben Brown and Joe Daniher were all in the negative.

Daniher's last positive WAR value was his All-Australian campaign three years ago – when he recorded 0.786 – while Brown was a fairly strong WAR performer in 2019 (0.499) and 2018 (0.658).

The value is cumulative, so injuries and missed games hurt footballers' overall WAR, although this can be broken down to average per game.

Players who, perhaps, outperformed their perception were Power general forward Robbie Gray (1.206), Eagles key defender Tom Barrass (0.954) and Dockers wingman Adam Cerra (0.828).

WAR's calculations piggyback on Champion Data's AFL Player Rating system and use a baseline measure to try to achieve greater equality between positions.

For instance, ruckmen must reach a higher AFL Player Rating baseline (10.3) than every other position, whereas general defenders (8.1) and key defenders (8.2) have the lowest.

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"If you look at AFL Player Ratings, you'll notice ruckmen are spitting out results that are probably higher than expected," Manoussakis told AFL.com.au.

"You wouldn't expect a midfielder you select in the rookie draft to turn into a player who's an All-Australian, for example, whereas that's within the realm of possibility for a ruckman.

"We're comparing the level of output for a player in the AFL Player Ratings to that replacement level and taking the difference, so, essentially, that's your value added."

One criticism of single-number metrics is that it over-simplifies player performance, while experts' inability to factor in off-ball actions is a limitation that hurts defenders most.

AFL player salaries not being public also prevents WAR reaching its full potential, but Roberts still sees plenty of positives.

"The main advantage of WAR, for me, is the ability to break down a scoring play into more than just who kicked the goal," Roberts said.

"It gives incremental value share to each player throughout the scoring chain that created the goal at the end. By doing that, we get a much better picture.

"Jonathan Brown could've kicked 100 goals, but it might've been on the back of incredible turnover work and he might be marking the ball 5m out with no one on him."

Follow Christos Manoussakis and Charles Roberts on Twitter to see more of their WAR-related work