The Professional Football Players Observatory was established in 2005 to monitor demographic trends in European football. It has just released its demographic study for 2011. This time the study has been extended to cover the top-flight of 36 national associations – all members of Uefa – covering over 13,000 players. It is, by some distance, the most detailed study of its kind.

Some of the data is interesting largely for curiosity value. The study shows for instance that the average European club squad comprises 24.5 players of whom 5.2 are aged 21 or under, 8.1 are over 1.85cm tall, and 8.2 are expatriates (1.1 of whom are Brazilian). Internazionale have the oldest first-team squad in Europe (29.61 years) and Olimps Riga of Latvia the youngest (19.02 years). SV Mattersburg of Austria and the Ukrainian side Volyn Lutsk have the tallest squads (186.68cm) and Barcelona the shortest (177.38cm). English clubs have the largest squads (27.4 players) and Czech clubs the smallest (22.1). English clubs have the highest percentage of internationals in their squads (61.1%) with Germany second on 49.6%, the Irish league has the fewest internationals, only 2%, and is the only one of the 36 leagues surveyed not to feature any Brazilians.

Where the study gets really interesting, though, is when it traces trends, often using the PFPO coefficient to rank national championships. "The coefficient," the report explains, "is calculated from the theoretical position of leagues on the regression line obtained by correlating the percentage of matches played in European club competitions by the representatives of national associations and the average personnel expenditure of clubs according to championship." England is thus given a value of 1.0, Spain 0.93, Italy 0.90, Germany 0.88 working through Scotland (0.49, ranked 14th), Poland (0.30, ranked 24th) to Estonia (0.09, ranked 36th).

Does size matter?

The average height of a footballer in Europe is 181.96cm, up 0.28cm on 2008, when PFPO first carried out their study. That suggests there could be some truth to the regular lament of former players that football is becoming more and more about physique and less and less about technique. The Premier League is the seventh tallest of the 31 leagues studied (five were unable to provide sufficient data), Scotland 27th.

Yet Barcelona, surely the best side in the world at the moment, have the shortest squad (followed by Shamrock Rovers, AEK Larnaca, Lorient and St Mirren). The liberalisation of the offside law logically should mean more space, and thus less of a premium on size – which is perhaps reflected in the fact that the PFPO study shows midfielders tend to be shorter than other positions. So why the rise? Perhaps at a lower level, physique continues to be prized. Perhaps Xavi, Lionel Messi, Andrés Iniesta, Wesley Sneijder, Samir Nasri and Luka Modric are a freakish generation of gifted short players who will soon be replaced by giants.

Or perhaps there has been a lag between changes at the top of the game and scouting systems catching up. The PFPO study shows an overwhelming tendency for the 11 most picked players in a squad to be shorter than their team-mates; it's only a theory, but could it be that when judging who to buy, height is far more of an issue than it is when coaches, having seen their players on a training pitch and had a greater opportunity to assess ability, are picking who to play?

How important is stability?

It's a commonplace of punditry when discussing whether a manager should be allowed more time before being sacked to point out that the two longest-serving managers in the Premier League are Sir Alex Ferguson and Arsène Wenger and to conclude that stability brings success. Of course the opposite is also true, that success brings stability and, while Manchester United's board showed great patience to Ferguson early in his career, and the Arsenal board are showing great patience to Wenger now, essentially both have stayed in their jobs for as long as they have because they keep winning things. And there are counter examples: José Mourinho has made a habit of club-hopping with significant success, while the great Bela Guttmann declared early in his career that "the third season is fatal" and made sure never to stay long enough anywhere to put his theory to the test.

The PFPO study shows a clear correlation between player stability and success – although again it would be a step too far to claim causality. Generally speaking, the higher the PFPO coefficient of the league, the longer a player will stay: Sweden top that list with an average stay of 3.19 years, with the Republic of Ireland last with an average stay of 1.73 years.

Manchester United is the most stable club, with players staying an average of 5.71 years (although it will be interesting to see what happens when Ryan Giggs, Paul Scholes and Gary Neville retire), followed by Dynamo Kyiv, Barcelona, SV Mattersburg and Hacken. Or, to add more statistical meat to the bones, in 24 of the 36 leagues studied players stayed longer at the four highest ranked clubs than the other sides in the league.

The PFPO study took Switzerland and Ukraine as case studies, plotting average stay of player against average points per game. In Switzerland the correlation is clear (a coefficient of determination [R2] of 46%); in Ukraine less so (R2 25%), with Shakhtar Donetsk winning more points than might be expected for the average length of stay of their players, and Metalist and Sevastapol fewer.

What's the impact of foreign players?

Blaming foreign imports for a national team's failings always seems a little simplistic, something that is probably seen better in cricket than in football. A decade or so ago (things began to change after the 2005 Ashes, and have shifted radically in recent months) it was common to see pundits lamenting the number of foreign players in the English county game after the Kolpak ruling, supposedly stopping young English players getting a chance. At the same time others insisted the county system was too baggy and that 18 counties meant that it was too easy for young English players to break through so they weren't toughened by the fight for recognition as their Australian counterparts were.

Both, clearly, couldn't be true and the suspicion now must be that – more by luck than judgment – a happy medium had been reached whereby young players found opportunities in a challenging environment (which makes the decision to cut the number of foreign players from two to one troubling).

In football, you would have thought that having 180-200 English players playing in the Premier League, by general consent one of the top two leagues in the world, would be a big enough pool from which to select a 23-man national squad. Only Cypriot clubs have a higher percentage (72.3%) of expatriate players (defined as "a footballer playing outside of the country in which he grew up and from which he departed following recruitment by a foreign club") than English clubs (58.4%), but when the percentage of expatriates is plotted against the PFPO coefficient (R2 42%), you find England actually has relatively few expatriate players (as do Spain and France). Cyprus and Hungary are the big outliers on the other side of the line – that is, relatively large numbers of expatriate players for the PFPO coefficient. Of the five countries with the highest percentage of expatriate players, England is the only one where that percentage has not grown in the past two years.

The PFPO figures aren't designed to show a correlation between the percentage of expatriate players in a league and the performance of the national team. Other factors such as wealth, size of population and footballing history must be taken into account before any realistic assessment of over- or under-achievement can be made. But, as a very crude measure, let's take the 10 leagues surveyed with the highest percentages of expatriate players: Cyprus, England, Portugal, Greece, Belgium, Scotland, Turkey, Italy, Germany and Russia. Their Fifa world rankings are: 89, 6, 8, 11, 58, 52, 31, 14, 3, 13. Now let's take the bottom 10: Serbia (the fewest expatriate players), Iceland, Estonia, Lithuania, Slovenia, Czech Republic, Latvia, Croatia, Bulgaria, Republic of Ireland. Their Fifa world rankings are: 23, 113, 86, 55, 17, 30, 76, 9, 49, 35. It's a very basic way of looking at it, but that surely is enough to suggest, at the very least, that the relationship between expatriate players in a league and international success is nowhere near as simple as some have made out.

• For more details on the PFPO study, see www.eurofootplayers.org. Jonathan Wilson can be followed on twitter at @jonawils