Women's College Liberos: Part II (Digging)

We began our analysis of the nation's top women's college liberos with Part I on serve receipt (plus an addendum). Today, we unveil Part II on digging. As I've noted before, the ideal way to evaluate liberos would be with quality ratings of each contact, such as difficulty level of each dig and whether the dig leads to a team running its offense in system. Lacking such fine-grained ratings, however, I must rely on publicly available, online box scores.

When looking at serve-receipt, I initially evaluated USC's Natalie Hagglund, Iowa State's Kristen Hahn, and Michigan State's Kori Moster. Then, based on suggestions from readers at VolleyTalk, I added three more liberos: Nebraska's Justine Wong-Orantes, Hawai'i's Ali Longo, and the University of San Diego's C'era Oliveira. I also have a local interest in Texas Tech's Rachel Brummitt, an excellent libero whose performances may go "under the radar" nationally. I concluded from the analysis of serve-receipt that, whereas Hagglund may consistently be the most flawless at the task, she handles relatively few of the Trojans' serve-receipt opportunities. By my formula, which accounts for both error-free receipt and share of the team's serve-receiving load taken on, Hahn and Wong-Orantes scored best.

Today, we examine the digging statistics for these same seven players and their teams. Whereas the conventional statistic in this area is digs per set (or game), we want to look more specifically at digs per opportunity. To remind everyone, according to the NCAA volleyball manual, "Digs are given only when players receive an attacked ball and it is kept in play..."      

When an opposing team attempts a spike, five types of results can occur, as shown in the following table. (Unless you have amazing eyesight, you'll want to click on the graphic to enlarge it.) The opposing team may score a kill. Or it may commit one of two kinds of hitting errors: getting the ball blocked right back onto the hitting team's floor for a defensive point, or hitting the ball out-of-bounds or into the net (for simplicity, I refer to such outcomes only as "out-of-bounds"). An attempted attack can also be dug; below, I list the digs both by the focal libero and the rest of the team (i.e., the libero's teammates). Finally, we have the "miscellaneous" category. This would include scenarios in which the defense blocked the spike attempt back to the offensive team, but the offensive team kept the ball alive.

To walk everyone through an example, USC's opponents collectively attempted 3773 spike attempts (as of last Sunday). Of these, 1267 or 33.6% resulted in kills for the opponents. A combined 15.7% of opposing spike attempts resulted in errors: 281 (7.4%) were blocked by the Trojans and 313 (8.3%) were hit out-of-bounds or into the net. A total of 43.9% of opposing attack attempts were dug: 552 (14.6%) by Natalie Hagglund and 1107 (29.3%) by Trojans other than Hagglund. Finally, 253 (6.7%) opposing spike attempts fell into the miscellaneous category.

Using Texas Tech as a second example, here is how the above information was gleaned from the team's online statistics page.

Note that the metric in the above chart is percentage of all opposing spike attempts dug. Iowa State's Kristen Hahn leads among the liberos studied at 16.6%. One out of every six spikes attempted by Cyclone opponents ends up being dug by Hahn! The chart also yields other interesting tidbits of information, such as which of the studied teams gives up the highest and lowest percentages of kills to their opponents, which teams block the highest and lowest proportions of opposing spike attempts, etc.

However, we can get even more specific. A player cannot dig an opposing spike attempt that is blocked at the net or hit out-of-bounds. Therefore, we use opponents' non-error attacks as the denominator representing possible dig opportunities (modified to also exclude balls blocked back to the attacking team and kept in play). The following chart includes the relevant equation (in red), which is applied both to teams and individuals.

Again, let's walk through some examples. Iowa State, as a team, dug 1508 balls. The number of opposing spike attempts the Cyclones conceivably could have dug is 2521 (the 1508 they actually dug, plus the 1013 opponent kills Iowa State theoretically could have dug, but didn't). Actual digs divided by conceivable digs (1508/2521) thus yields .598 or 59.8%, as shown above.

What about individual liberos? If we had spatial data on regions of the court to which spike attempts were directed and an idea of how much ground the libero (and other players) was expected to cover, we could compute an extremely precise ratio of a given libero's digs over how many spiked balls she could have dug. Lacking the spatial information, however, I made the less realistic assumption that a libero would be expected to dig all of the opposing teams' spikes that went either for kills or digs. In Kristen Hahn's case, she had 524 digs. Dividing 524 by the 2521 attacks the entire Cyclone team conceivably could have dug (calculated above) yields .208 or 20.8%. All of the liberos studied were subject to the same assumption, so I think the resulting percentages (ranging from Hahn's 20.8 to Justine Wong-Orantes's 12.7) are comparable to each other.

These results should be interpreted with caution, for a few reasons. First, teams do not play equally strong opponents. Comparing teams and players within the same conference is probably reasonable, although I used teams' overall records, not just conference ones. Second, volleyball involves a lot of interlinked, moving parts. For example, when a team blocks a large share of its opponents' spike attempts (e.g., Nebraska and Michigan State, which each blocked slightly more than 8% of opposing attacks), it could mean that many "dig-able" balls are getting stopped at the blocking stage, with more challenging spikes getting past the block and making things harder for the backcourt players to dig.

One final note: Texas Tech coach Don Flora made digging a key priority this season. As shown in the last table, the Red Raiders dug 55.3% of the conceivably "dig-able" balls hit their way. A year ago, the comparable figure for Texas Tech was 50.9%: 1684 TTU digs/(1684 TTU digs + 1624 opponents' kills).