EFFECT OF ARTIFIALTURF VS. GRASS

written by David W. Smith
June 17, 1995

turf


The effect of natural grass vs artificial turf on batting performance has been debatedsince the ersatz material was first installed in the Astrodome in 1966. Many differentkinds of arguments have been advanced, supported by various types of data.

The present analysis is of all the 22,806 games playedfrom 1984 through 1994, covering over 1.5 million at bats. The play by play dataused here came from the Baseball Workshop in Philadelphia, from which anyone canorder them.

The most common argument put forth is that battingaverages are inflated by artificial turf because the ball moves so much faster andtherefore ground balls get through the infield more easily. An examination of thissimplest assertion appears in Table 1.

Table 1.

Batting average by league and playing surface, 1984-1994.

League Surface Games At Bats Hits BA
AL Grass

8664

590960

154545

0.262

Turf

3466

237665

62888

0.265

NL Grass

5474

372243

95367

0.256

Turf

5202

352519

89421

0.254

Both Grass

14138

963203

249912

0.259

Turf

8668

590184

152309

0.258

Several interesting conclusions may be drawn fromthis table. First of all the two leagues play very different numbers of games onthe two surfaces. In the AL there are 2.50 games on grass for every one on the carpet;in the NL the figure is 1.05.

The AL has an overall higher batting average of .263to .255. However the differential as a function of surface is quite small, with theAL having a three point increase on turf and the NL a perhaps unexpected two pointdrop on the artificial surface. The league differences cancel out, leaving a netof .259 in all games on any surface.

The information in table 1 is really much too superficial,however, since different parks differ in many respects other than the playing surface.For example, all domed stadia have artificial surfaces and domes have been suggestedas significant factors in offensive statistics. Some other factors that affect battingaverage besides playing surface are: symmetry of the playing field, extent of foulground, height of fences, prevailing wind conditions, average temperature, and altitude.

The analysis was therefore refined in an attempt tominimize these other effects (the operative word is clearly "minimize",since complete elimination of all effects besides playing surface is a very elusiveobjective). The refinement done here is to modify and expand the batting averagecalculation. Batting average may be defined as the number of successes per opportunity,where the successes are safe hits and the opportunities are at bats. These two basicparameters were adjusted in four ways:

1. Subtract home runs from hits and at bats. Ballshit over the fence presumably are affected very little by the nature of the playingsurface.

2. Subtract strikeouts from at bats. Since strikeoutsare plays in which the ball is not contacted, the effect of the playing surface canbe safely ignored.

3. Add sacrifice flies to at bats. Sacrifice fliesare currently not included in at bats (for many years they were), although they areessentially regular fly balls that are not hits.

4. Subtract non-sacrifice bunts from hits and at bats.Although there is very likely an effect of the playing surface on the chance of asuccessful bunt, the primary analysis is concerned with balls put into play on fullswings. Sacrifice bunts are not at bats, but the data used do identify bunts, sobunt hits and bunt outs that are not sacrifices are removed.

Net Effect:

Adjusted At Bats = At Bats

+ Sacrifice Flies

- Home Runs

- Strikeouts

- Bunt Hits

- Bunt Outs

These adjustments can be summarized simply: all ballsput into play on full swings are considered and only balls put into play on fullswings are considered. In this context "in play" means that a fielder couldconceivably make a play, therefore the exclusion of home runs. The small number ofinside the park home runs (10-15 per year) is disregarded. In addition each typeof hit: single, double, or triple, is considered separately, since there are goodreasons to believe that they will be affected differently by the playing surface.

The analysis will be presented in three phases: 1)all games for the 11 years in aggregate; 2) all games for individual seasons; 3)all games for individual teams for each season. Moving to smaller subsets of thedata, such as the performance of individual batters, tremendously increases the statisticalvariability and is more likely to confuse the analysis than to enhance it.

All Games for 11 Seasons

By making the adjustments described above and separatingeach hit type, we obtain the information presented in Table 2. For both leagues individuallyand combined the patterns are unmistakable:

the rate of singles goes down on turf about 3.9% whilethe rates of doubles and triples go sharply up, 13% and 36%, respectively. The combinationof these values into an adjusted batting average (labeled "ABA" in thetable) shows a small increase in the AL for games on artificial surface and the reverseeffect for NL contests. These two results balance each other so the net consequenceis no difference for the Majors overall.

Table 2.

League Surface Games Singles Doubles Triples ABA
AL Grass

8664

0.228

0.055

0.00652

0.290

Turf

3466

0.221

0.064

0.00933

0.294

NL Grass

5474

0.230

0.053

0.00718

0.290

Turf

5202

0.219

0.060

0.00909

0.288

Both Grass

14138

0.229

0.054

0.00677

0.290

Turf

8668

0.220

0.061

0.00919

0.290

All games for individual seasons

The next approach is to examine each league for eachindividual season, giving us 11 comparisons for each league. Rather than report amassive list of actual rates, table 3 contains a list of the number of times thateach category increased in games on artificial turf out of the 11 seasons for thatleague and how many times that category went down in games on turf. For example,in the 11 AL seasons, the actual batting average was higher on turf 8 times. In theNL the increase occurred in only 4 of the 11 seasons. The individual hit categoriesare completely consistent in the surface effect they show: each league in each seasonhad a higher adjusted rate of singles on grass and higher adjusted rates of doublesand triples on turf. Combining these three into the adjusted batting average givesproportions similar to the actual batting average changes.

Table 3.

Proportion of increases on artificial turf in ratesof actual batting average, rates of singles, doubles, triples and adjusted battingaverage for all seasons, 1984-1994.

League BA Singles Doubles Triples ABA
AL

8/11

0/11

11/11

11/11

9/11

NL

4/11

0/11

11/11

11/11

4/11

Individual team-seasons

The last stage of the expansion is to consider thenumber of times each individual team had its rates increase on artificial surfacein a given season. For the AL there were 154 team-seasons from 1984-1994 and forthe NL there were 136. These results are presented in table 4, which is parallelto table 3 in its organization.

Table 4.

Proportion of increases on artificial turf in ratesof actual batting average, rates of singles, doubles, triples and adjusted battingaverage for individual teams for all seasons, 1984-1994.

League BA Singles Doubles Triples ABA
AL (all teams)

82/154

50/154

115/154

109/154

87/154

NL (all teams)

53/136

30/136

104/136

94/136

52/136

Although the results shown in Table 4 are consistentwith the earlier information, we see that increased variability is introduced whenthe data sets get smaller. One conclusion to be drawn here is that the response ofa given team in a given season may be contrary to the overall trend without seriouslycompromising that larger picture.

Overall conclusions

1. There is no consistent change in the rate of hitsper adjusted at bat due to artificial surface. There is a slight increase in theAL, which is balanced out by a slight decrease in the NL.

2. There is a striking change in the distributionof type of hit on artificial surface. Singles are consistently lower on the turf,but doubles and triples are dramatically increased.

Explanations

Any attempt to offer a mechanism to account for theseresults must directly consider observable player responses to the two different surfaces.Let us return to the initial premise: the ball moves faster on artificial surface.That assertion seems to be indisputable, not only by observation from the sidelines,but also by listening to the men who play the game. However, the consequences ofthis increased speed of ground balls is less obvious. The first thought might bethat more balls will get through because of the greater speed. However, infieldersclearly play several steps deeper on turf than they do on grass. This greater depthis justified even though it means they have a longer throw to first base becausethe greater speed with which the ball reaches them means that they have more timeto make the throw. When an infielder plays deeper, his lateral range is significantlyexpanded, allowing him to get to more balls than would be possible on grass. Furthermore,when infielders play deeper, they are more able to catch bloopers over the infield(so-called "Texas-Leaguers"). I know of no study which charts the positionof the fielders when each ball is put into play, but I am convinced, based on myown anecdotal observations, that the pattern suggested here is true. I have watchedmany games in Philadelphia (turf) and Memorial Stadium in Baltimore (grass), andthe distance between the infielders and outfielders as they await a pitch at thetwo parks is noticeably different, with a much greater gap on the grass field, eventhough the overall park dimensions are similar and the outfielders are playing nocloser to the fence in Baltimore.

If the infielders do alter their positions as suggestedhere, is there are any way to examine this data set for evidence of it? The bulkof this analysis has involved safe hits, which are of course a minority event. Itherefore went through every play of every game and tabulated the number of timesballs were hit to infielders on outs, including errors and fielder's choices whereno one was retired as well. Furthermore, this tabulation was subdivided into ballshit on the ground and balls fielded in the air, the latter category including popups and line drives.

Finally, since there is a chance that varying amountsof foul ground will have an effect, foul balls, either caught or dropped for errors,were also tabulated. The results of this analysis are in Table 5. Rates are calculatedby dividing each value by the adjusted at bats with foul balls removed.

Table 5.

Rates of foul balls and outs fielded by infielders,separated by ground balls and balls hit in the air.

League Surface Foul % Ground % Air % Infield %
AL Grass

0.032

0.280

0.078

0.359

Turf

0.033

0.284

0.077

0.361

NL Grass

0.034

0.288

0.077

0.365

Turf

0.031

0.286

0.078

0.364

Both Grass

0.033

0.283

0.077

0.361

Turf

0.032

0.285

0.077

0.363

These results show clearly that the surface does nothave an effect on the number of balls fielded by infielders, which would appear tobe in contradiction to the claim asserted above. However, it is more likely thatthese numbers show that the fielders have altered their position on the field tomaintain an optimum chance of getting to a ball.

Infielder positioning was posited as an explanationfor the decrease in singles on artificial turf. The extra base hits require a differentanswer. Since the ball moves so much more quickly on turf, a ball which gets throughthe infield will have a much greater chance of getting past the outfielders as well.Although it is highly anecdotal, I clearly recall a game in Philadelphia in the late1970s when Larry Bowa, the Philadelphia shortstop, just missed a ground ball to hisleft, sprawling on his face in a futile dive. The ball rolled to the wall in leftcenter and the batter got a triple. It is highly unlikely that such a ball, evenif had gotten through the infield, would have become a three base hit on a grassfield.

Conclusion

One additional category worthy of a brief considerationis bunts. All bunts (sacrifices plus bunt hits plus non-sacrifice bunt outs) occurabout 17% more often on grass fields than on turf. Since a bunt is an intentionalplay on the part of the batter, one must consider that this significant differencein occurrence reflects player perceptions of the difficulty of bunting on turf.

There is an expected, although slight, differencein the success of non-sacrifice bunts as well, with 39.9% of them being hits on grassvs 36.6% on artificial surfaces. Although artificial surface seems to have only aminor effect on batting average, there is a clear effect on the pattern of safe hits.One might expect that the increase in doubles and triples on artificial turf wouldoffset the lower rate of singles and lead to overall higher offense on the carpet.However, as shown in Table 6, scoring on grass is actually slightly higher (8.82runs per game for both teams) than on turf (8.62 runs per game for both teams) duringthe past 11 seasons, although the two leagues differ in the direction of the change.

Table 6.

Runs scored as a function of league and surface.

League

Surface

Games Runs Runs/Game
AL Grass

8664

78629

9.08

Turf

3466

31590

9.11

NL Grass

5474

46053

8.41

Turf

5202

43081

8.28

Both Grass

14138

124682

8.82

Turf

8668

74671

8.62

Note added in April, 1996. The analysis inTable 6 has a methodological error in that home runs are not excluded. The properadjustment would be to subtract the number of runs attributed to home runs so thatthe effect of balls hit in play on scoring could be seen more clearly.

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