Does Saprissa's artificial pitch give them a greater home advantage?

Most people would say that CD Saprissa's artificial turf gives them a significant home advantage.  But Saprissa's stadium has always been intimidating to visiting teams in the days when it still had a natural grass surface.  Assuming everything else equal, how significant is the difference in match results from the grass pitch and artificial pitch eras? 

To investigate this problem, I collected the home match results for Saprissa over the past ten seasons, from the 1998-99 season to the 2009 Invierno tournament.  (I disregarded matches played after the first of this year for reasons that I'll explain later.)  I divided the data into two parts: the period between 1998 and the spring of 2004, when Saprissa Stadium had a natural grass surface, and the period between the fall of 2004 and the present, when Saprissa had their an artificial surface.  A new surface was installed at the end of last year, but not enough matches have been played on it to allow meaningful analysis separately.  I further grouped matches that were played in the Primera División from those played in the UNCAF Nations Cup or the CONCACAF club competitions (Champions' Cup, Champions League, and Giants Cup).

So here are the home match statistics for Saprissa, before and after the conversion to a synthetic pitch:

	Domestic		International		Total
	Natural	Artificial	Natural	Artificial	Natural	Artificial
GP	149	112	7	26	156	138
Wins	112	67	4	17	116	84
Draws	23	31	3	6	26	37
Losses	14	14	0	3	14	17
Goals Scored	352	200	10	53	362	253
Goals Allowed	125	92	2	25	127	117
Points	359	232	15	57	374	289
Points/Gm	2.41	2.07	2.14	2.19	2.40	2.09
Points %	80.31%	69.05%	71.43%	73.08%	79.91%	69.81%

Over all of Saprissa's home matches their performance has worsened in terms of points taken and the number of goals scored per match.  Naturally, their overall performance is dominated by their domestic home record; if you separate the games played in Costa Rica's Primera from those played in international competition, there appears to be little difference in their international record since the field conversion.  (Saprissa played too few international matches before the fall of 2004 to make any solid statistical conclusions.)

If you look at the goals scored and allowed statistics, Saprissa's per game record looks like this:

	Domestic		International		Total
	Natural	Artificial	Natural	Artificial	Natural	Artificial
GS/Game	2.36	1.79	1.43	2.04	2.32	1.83
GA/Game	0.84	0.82	0.29	0.96	0.81	0.85

The results are quite surprising.  In domestic home matches, the opposition has not found it harder to score goals since the field conversion -- the average number of goals scored per game has decreased very little.  What is astounding is that Saprissa's goal scoring rate has plunged 24% since the conversion.  That's a really significant amount, and it's swung the goal differential from +1.52 on a natural surface to +0.97 on the artificial.  The results in international competition are even more odd.  Opposing teams are scoring more goals per game than they were before the field conversion.  But Saprissa's goal scoring rate has increased 42% in international matches since the conversion to synthetic turf, which has kept the goal differential about the same (+1.14 on natural versus +1.08 on synthetic). 

The expectation is that by separating the international and domestic matches, one is in effect controlling for team quality, and Saprissa has always been among the Big Two in Costa Rica so one would expect a better performance in domestic league matches than international.  But I think there might be an explanation for these results. 

Saprissa's league opponents play Saprissa a lot more often than CONCACAF teams, and they are more familiar with playing on a synthetic surface (there are at least three or four Primera clubs using such fields).  It may not help these clubs in terms of scoring on Saprissa's field, but it could help them in terms of defending -- they are more familiar with how to exploit the bounces to defend, they might stay vertical more often when defending, and so on.  Few professional clubs outside of Costa Rica have artificial pitches, which not only inhibits their ability to score but also to defend.  Now, these clubs are of higher quality than the domestic league clubs, so they will score more goals on average.  But they may not defend as aggressively at Saprissa as they might at home.

Finally, I want to highlight Saprissa's home record against the other member of the Big Two, Alajuelense:

	Alajuelense
	Grass	Artificial
GP	12	13
Wins	5	8
Draws	2	4
Losses	5	1
Goals Scored	15	18
Goals Allowed	11	7
Points	17	28
Points/Gm	1.42	2.15
Points %	47.22%	71.79%

Now, both teams were out of phase in the two periods listed here -- Alajuelense were winning championships in the late 1990s and early 2000s while Saprissa were in a low period, and then Saprissa started winning titles in the mid-2000s during Alajuelense's lean period.  But even accounting for that, Saprissa's home performance against their biggest rival improved substantially after the move to artificial turf.  It's interesting to note that Alajuelense's lone win in Saprissa occurred after the team has converted its own stadium to artificial turf.

So to conclude, Saprissa's artificial pitch appears to have hurt its home advantage at this time, not because opposing teams have scored more goals, but because Saprissa's own players have accustomed themselves to it sufficiently to score goals of their own.  But in international competition, it appears that visiting teams are unfamiliar with the challenges of defending on a synthetic field, and Saprissa have a better goalscoring record than they do in their own domestic competition.

Further analysis that includes a look at how the goal distribution at Saprissa matches have changed is posted at my companion website Soccermetrics.  I got a little carried away with the analysis, so here is Part I.  Part II will be posted at a later time.