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Value for money: Dusan Tadic, Diafra Sakho and three January comparisons

Diafra Sakho and Dusan Tadic have been superb in the Prem since joining in the summer.

Now that the January transfer window is open, the next month could see a few of the world's best players join Premier League teams. If nothing else, it will almost certainly produce plenty of stories linking most of the world's players in moves to England. It's nearly impossible to track every single footballer linked to a possible transfer, so in these frantic, rumor-driven times statistics can be extremely useful.

To explain how they help, consider two of the most productive transfers of the summer window. West Ham United purchased Diafra Sakho from Ligue 2's FC Metz for under four million pounds (six million dollars) while Southampton snatched Dusan Tadic from FC Twente for about 10 million pounds (17 million dollars).

Both players came in from smaller leagues, and both have thrived. Sakho scored his eighth goal of the season on New Year's Day, and his per-minute rate of goals scored (0.67 non-penalty goals per 90 minutes) trails only Sergio Agüero and Diego Costa.

Tadic scored last Thursday as well. The goal was just the Serbian's second, but he has notched eight assists in 18 league matches. This gives Tadic a rate of about 0.6 non-penalty goals and assists per 90 minutes. Among Premier League midfielders, Tadic stands in impressive company.

Tadic was a cheap pickup for the Saints yet produces at the rate of the Prem's most high-priced stars.

Most of the players surrounding Tadic on the above chart cost more than double what Southampton paid in transfer fees for their new playmaker. He looks like a fantastic acquisition as Tadic's performance was predictable according to the underlying stats. (Sakho's might be less predictable, and I will return to him later.)

There are, of course, many players who rank up impressive goal and assist totals in less competitive leagues. What marked Tadic out for success are the shots that he either attempted or assisted, rather than the goals themselves. This focus on shots stems from one of the key insights that football statistics provide.

In short, the rate at which chances are scored is highly variable. One season, Papiss Demba Cissé might score just one of his 48 shot attempts; the next he could score eight of 27. A recent example that bears directly on the transfer market is Ciro Immobile. Borussia Dortmund purchased the striker from Torino for about 15 million pounds ($23 million) after a season in which Immobile scored 22 non-penalty goals on 101 shots. For Dortmund, Immobile has scored just three goals from 30 attempts this season, a rate less than half what he managed for Torino.

Tadic is unique because his obvious goal and assist contributions match his overall output.

Case studies like Immobile and Cissé aren't that unusual, either. Year to year, players have almost no consistency in the rate of shots they score. Because of this variability in chance conversion, it is more instructive to look at a player's full shooting record. The ability to get into good positions for chances or in which to set up chances for another player, is much more consistent year-over-year when looking at the statistical record.

Now, this does not mean there is no such thing as finishing skill. It is clearly the case that some players have a more deadly shot than others. However, to identify a good finisher takes a sample of several hundred shot attempts. Over even 50 attempts, a perfectly good striker like Cissé can run cold and see his skill overshadowed by that short-term slump. If you projected Cissé's goal-scoring record this year on the basis of his 2013-2014 season, you would judge him incorrectly so unless a player has an extensive and consistent playing record, it is best not to put too much weight on actual goals and assists in isolation.

To gauge the quality of a player's shots and chances created, I have built a statistic called "expected goals." This measurement considers the shooter's location on the pitch, what kind of pass assisted the shot, the speed of the attacking move that led to the attempt as well as several other factors. Expected goals and expected assists are not subject to the same kinds of fluctuations as simple goal and assist stats, and they can provide a clearer picture of the player.

In the Eredivisie, Dusan Tadic not only scored nine non-penalty goals and assisted 14, but he also attempted 82 shots and assisted a further 129. These shot attempts were not simply a passel of speculative long-range efforts, either; nearly half of the shots Tadic assisted came from the "danger zone," aka the center of the 18-yard-box from which most goals are scored. He had about 7.5 expected goals and 15.5 expected assists, showing that his underlying statistics match up well with his top line numbers.

Furthermore, Tadic was heavily involved in Twente's attack beyond just attempting shots. I built a statistic for involvement in build-up play which measures how often a player contributes to the open-play attacking move that led to a quality chance. If he completes a pass or a cross, or wins the ball with a tackle or interception in the build-up to a shot, he gets credit for being involved. Tadic led the Eredivisie in expected goals chance involvement by a significant margin.

Tadic doesn't just provide goals, but has contributed plenty to his team's overall attacks.

When Southampton purchased their Tadic they were not only buying a player with impressive goal and assist numbers, but a player whose underlying stats reflected his consistent contributions to his team's attack. When scouting for bargains, this sort of well-rounded contribution to the attack is precisely what you want to look for.

With that in mind, I've identified three players in particular from the Eredivisie and Ligue 1 whose numbers suggest that like Tadic, they have complete games that will translate against higher competition.

But first, a quick word on Sakho. Trackable data on Ligue 2 is limited and Sakho's explosion was as unexpected for statistical analysts as for anyone else. While statistics can be useful, there is as yet no direct equivalent to a great scout. Still, statistical analysis can identify players who may be overlooked, or who may be overrated.

1. Hakim Ziyech, FC Twente

This summer, Twente needed a replacement for Dusan Tadic after selling him to Southampton. They chose Ziyech, a 21-year-old playmaker at Heerenveen and he appears to have been an excellent choice.

Ziyech ticks almost all the boxes that Tadic did but is four years younger. Over the past two seasons, Tadic and Ziyech are the only Eredivisie players with significant minutes to have been involved in the buildup of over half their clubs' expected goals. Tadic stands top with 55 percent involvement; and Ziyech is second at 51 percent.

Furthermore, he has scored five goals and assisted four this season while his expected goals fall in the same range. Twente lost their best attacker in Tadic, but with Ziyech as his replacement, the club stand only three points off third place this year after finishing third last season.

If a Premier League club is looking to reproduce the Tadic transfer, they could do worse than targeting the kid who literally replaced him. Everton were rumored to be interested in Ziyech last summer before he want to Twente, and so it's possible that another economically under-powered club could consider him a bargain.

2. Paul-Georges Ntep, Stade Rennais

There is no clear Ligue 1 analogue for Ziyech but Ntep is notable is his own right. With six goals, three assists and a ninth-best 0.50 xG+xA per 90 minutes, the 22-year-old has provided an excellent end product. And like Tadic and Ziyech, he is also a complete contributor even when he is not scoring goals.

Ntep boasts 56 percent chance involvement in Ligue 1, better even than Tadic managed last season. That means that while Ntep has been on the pitch, he has contributed to the build-up of more than half of their good open-play scoring opportunities. It's notable given that he is a distinctly different player. Ntep is a winger or wide forward who moves play forward quickly. His combined abilities to shuttle possession and break out a dangerous final ball either from the wing or cutting inside have made him indispensable to his current club.

Ntep is in Ligue 1's top 10 this season in terms of chance involvement and chance conversion.

With 0.50 xG+xA per 90 minutes, Ntep stands tenth in Ligue 1. The French speedster has been linked to moves to Stoke City or Tottenham Hotspur at a price in the range of 6.5 to 10 million pounds ($10-15 million), Ntep's numbers suggest he would be a smart purchase at that price.

3. Alexandre Lacazette, Lyon

Lacazette has been the breakout star of Ligue 1 this season, scoring 17 goals including three penalties and carrying Lyon to second place at the season's halfway mark, a point ahead of giants PSG. The French striker may, however, be on something of a deceptively hot streak in his finishing.

The following chart displays all the shots attempted by Lacazette and Ibrahimovic, with goals marked in black.

Lacazette has Zlatan beat so far this season, but it's an unsustainable pace.

Lacazette has scored 14 goals from chances that have an expected goals rating of only about seven, a conversion rate about twice as efficient as Zlatan Ibrahimovic. (Or to put it another way, Ibrahimovic has produced similar expected goals to Lacazette in half as many league minutes.) That is a tough act to maintain.

This means that unlike Ntep, Ziyech and Tadic, Lacazette's underlying stats do not line up with his top-line production. If Arsenal or Manchester City match Lyon's likely asking price of 20 million pounds ($30 million) or above, they will be taking a notable risk as spending big on Lacazette means betting big on a player to significantly outperform his expected goals stats over time.

While top strikers do beat their expected goals, even Lionel Messi is no better than 40 percent above xG. Strikers with conversion rates that dwarf those or Zlatan or Messi are almost always headed for a fall.

Michael Caley is a writer bringing a statistical approach to football analysis. His work also appears at SBNation, the Washington Post. Twitter: @MC_of_A.

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