ZvX: The Effects of Scouting Pillars

Having a scouting pillar for Overlord is considered one of the unspoken rules in modern map design. Does the scouting pillar affect the outcome of the game? This article examines this question.

Background

This article is inspired by a post on reddit. The scouting pillar is the high ground outside of the natural expansion that allows Overlord to provide vision (see image below). 

It is a common feature in modern competitive maps. The main purpose of the scouting pillar is to provide a safe position for the Overlord to park outside the opponent’s natural, so the Zerg player could have vision of the natural. This allows the Zerg player to send the Overlord into the opponent’s base at the optimum time for scouting. As highlighted by the author of the reddit post, Wardi’s comment in the video below sums up the perceived implications of the scouting pillar. 

The author looked at the win rate across different maps descriptively and concluded that the scouting pillar has no effect on the match outcome. While I believe the question is interesting, the analysis has much room for improvement. For instance, any suitable multivariate analysis would provide better insight than the descriptive statistics alone. With this in mind, I decided to revisit this question with Ben, who has previously co-authored the article about the hidden side of GSL tournament format. He did all the tedious work with data and statistical analyses. All hate credits go to him.

Does scouting pillar affect the Zerg match ups? If so, how?

We did not have specific predictions. I personally would expect the presence of pillars to favour Zerg slightly. 

Data

We only considered non-mirror 1v1 games with Zerg that were played during Legacy of the Void, starting with balance patch 3.8 (implemented December 2016) to 5.0.2 (August 2020). Expanding our sample beyond the current patch to include games over close to 3 years does not just enlarge our sample size, but it also helps to minimise the influence of exogenous variables in the statistical models (e.g., patch, metagame trend, and other map features). This would allow us to better capture the effect of the scouting pillar on match outcome.

We collected data from sc2replaystats. Sc2ReplayStats is a Starcraft 2 Replay Hosting/Training System based around Custom Statistics. The dataset includes 5,239,543 games using 70 unique maps over 13 balance patches: 3.1.1 to 5.0.2. They range from all skill levels and are user submitted games. It includes games played in all regions and the gamut of Bronze to GM. Games that ended in ties or without definitive wins and losses were omitted. After filtering for Zerg games, we are left with a total of 3,572,576 ZvT and ZvP games.

Balance patch 4.1.4 has problematic data because it ran for less than two months before 4.2.1 was then released. Rows with less than 100 games played were also removed, so they don’t disproportionately affect proceeding calculations.

We initially categorised the maps into “no pillar”, “pillar can see the entrance of the natural”, and “pillar cannot see the entrance of the natural”, but we decided to combine the “no pillar” and “pillar cannot see the entrance of the natural” into a single “no pillar” category. This is because of the low number of submissions for “pillar cannot see the entrance of the natural”. Logically, if the benefit of the pillar is to provide vision for the Zerg as argued by many, then “pillar cannot see the entrance of the natural” should be treated as “no pillar” instead of “pillar can see the entrance of the natural”.

Furthermore, we wanted to study whether there was a pillar effect over time or over patches, but the number of maps without pillars lessened over time. 

2017: 7 of 20 maps did not have pillars

2018: 6 of 24

2019: 3 of 19

2020: 1 of 18

Four maps in the past 2 years and 4 balance patches does not provide enough of a sample to fairly compare sample sizes. It is worth pointing out that there are notably more maps with pillars than without pillars (2,911,923 vs. 660,653). Only two in the top 15 most played maps do not have pillars, and this reflects players’ veto preferences.The data also suggests that maps without pillars were becoming less common over the years.

Analyses

The table below sums up the win rate of Zerg for non-mirror Zerg games.

At first glance, the win rates appear to suggest that the scouting pillar has no effect on win rate in both match-ups because they are very close to 50%. However, given our hypothesis, 50.45% was used instead of 50% as the base win rate, because we are comparing whether having the pillar changes the win rate. 50.45% is the Zerg win rate on maps with a pillar. If having a pillar indeed affects win rate, we should see a significant difference. Further, the use of global Zerg win rate is not suitable, because the mean is derived from the sample per se. When analysing match-up specific effects, we pivoted to using the match-up specific pillar win rate as the basis, 51.77% (ZvP) and 49.35% (ZvT).

Using two-sided t-tests, the results showed that the presence of a scouting pillar does not affect the win rate (p = .214). To unpack the effect of a scouting pillar in each match-up, we repeated the process with only the ZvP games and only the ZvT games. The presence of a scouting pillar has no effect on ZvT (p = .526). However, the difference in win rate between maps with pillars and those without pillars for ZvP is marginally significant (p = .073, d = .334). Zerg has a win rate of 51.99% win rate on maps with pillars, but that drops to 51.28% on maps without pillars.

Discussion

The results suggest the effect of a scouting pillar on match outcome is more complex than one might assume. The presence of scouting pillars does not have an unwavering effect across all games, as it affects ZvP but not ZvT. I am sure some would be quick to point out that the difference of 0.7 percentage points in ZvP is negligible. However, it is important to highlight that the maximum difference in win rate between the best and the worst Zerg maps is just 4.5 percentage points. We should not underestimate the significance of the difference in win rate for the presence of pillars in maps.

Clearly, the results do not allow us to infer the exact reasoning behind the discrepancy across the match-ups. One plausible explanation is the relative value of scouting in the early game. The results imply that the information gathered via the Overlord on the scouting pillar has greater implications on Zerg’s decision making in ZvP than in ZvT. This suggests two plausible reasoning. One is that Zerg rely more on other ways of scouting in ZvT than in ZvP. Zerg would want to see the add-ons on the production buildings and selective tech buildings (e.g., Armory), as that helps to deduce the builds tremendously. The information is better gathered with Overlords going into Terran’s base from the side. The other plausible reasoning is that, compared to ZvP, ZvT requires Zerg to make less dedicated adjustments in the early game.

The effect of the scouting pillar might also be indirect. Players may pick certain builds on maps without the pillar. That is, the outcome could be due to the choice of builds and not the actual availability of scouting information itself.

Does metagame influence the effect of scouting pillars on outcome? And how? Most people would intuitively say yes, but I doubt anyone can back up their arguments with empirical evidence. We thought of using patches as a proxy for metagame in the analyses. Ideally, the results would inform us that the effect of scouting pillars is stronger in some patches (or periods) than in others. We could then, based on this information, deduce more precisely how scouting pillars affect ZvP. However, the limitation of the data makes it difficult to make comparisons across patches, because there are patches that have one map with no pillar. The problem goes beyond just sample size, as the comparison would then be map specific instead of patch specific. Thus, we did not conduct patch specific analyses.

This article is published at the best and worst time. Blizzard did not rotate the map pool for the latest season, and it is unclear if new maps will be introduced next season. Mapmakers’ interest in creating new StarCraft II maps is probably at its new low, so map improvement related discussion is rather meaningless. Conversely, this also implies that the current understanding of map design in StarCraft II has reached its peak, because the ladder maps are unlikely to be replaced by better maps. This would suggest that we have been making assumptions about the impact of certain map features that may be overly simplified or not hold. The use of machine learning could potentially lead to better and more interesting maps.

While our work shows some unexpected results, it is not without limitation. First, the difference in sample size for maps with pillars versus without pillars is obviously an issue for statistical inference. Second, the sample games from Sc2ReplayStats have selection bias. We could not access games that were not uploaded to the website, and there could be unexplained factors that contribute to the likelihood of uploading replays. For example, those who upload replays may be more serious about improvement than those who do not, and this could potentially affect map veto decisions and the choice of games to upload. Third, which is related to the second, the selected games were played by “average” players on ladder. It is unclear if the findings are generalisable to the pro level. We initially wanted to sample games from GSL, because it covers every patch. But the number of games for maps without pillars is simply too low.

Special thanks to those on r/allthingsterran who helped to code the maps. Also, big thanks to Henry for the huge Patreon support!


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4 thoughts on “ZvX: The Effects of Scouting Pillars

  1. Great article again as always!

    I think a potentially overlooked factor is the inevitable loss of the scouting overlord. At the lower ranks, scouting overlords are often lost without gaining significant value. For example, the overlord may die without the zerg player seeing the killing unit, or the killing unit/s is seen but the zerg player does not respond appropriately.

    In these cases, the scouting pillar may actually be detrimental to the zerg player instead. In addition, the timing at which this happens is important also. It may shed some light onto the differences in the zvt and zvp matchup. For instance, the first starport unit might give away less information than the first stargate unit.

    These are all conjecture, but these were some thoughts that i had. Again, thanks for continuing to provide content even when starcraft’s future is looking bleak :(

  2. I am not sure why you think that interest in creating new maps is at an all-time low. We are actually still designing new maps, whether or not there will be any sort of map contest as there has been in the past in order to introduce them. Moreover, your statement that “current understanding of map design in StarCraft II has reached its peak” is unfounded and baseless, as it relies on an assumption that the map pool will no longer be rotated and that there will no longer be new maps created for ladder. The truth is, you (and I) cannot know whether or not the map pool will be rotated; there is no assumption in either direction that can be reasonably made. The truth of the matter is that the map pool is continuing to be rotated in WarCraft III as well as StarCraft (Brood War), so if anything, it is likely that the map pool will continue to rotate in StarCraft II.

    There is still a lot to learn about maps, and there are still improvements that we can make in future maps. The understanding of maps and how the races play on them is certainly not maximal. As long as the meta can shift, so can the design of maps.

  3. Something you might have overlooked is the scouting pillar in ZvT is really not that important. More often than not, zerg’s first overlord is sent around the map looking for proxies, with it only being sent straight across if the zerg player expects a 3 rax reaper (this lets you see the barracks on the low ground/lack of a cc in time). What this means is, the overlord is delayed enough by the scouting pattern that they cannot reach the pillar in time if their opponent were to open marine first and send it to catch this overlord. This is why normally with this scout pattern, the overlord is sent somewhere adjacent to the terran base but almost NEVER on the overlord pillar. This still allows for a 4:00 scout to see the terran’s infrastructure.

    Conversely, in ZvP, the pillar has a much larger effect. One of the primary ways of zerg determining protoss’ build order is examining the units protoss makes from their first gateway. On maps with a pillar, this is very easy to do as there is no risk of the zerg player missing a unit coming out. However, on maps without a pillar, it is impossible to even see the first unit the protoss makes. Were the zerg to keep their overlord close enough to see the first unit, it would most definitely die were that unit a stalker first. Similarly, zerg cannot tell whether their opponent is going for 2 fast adepts which means they can freely assume the protoss is playing stargate and sacrifice their overlord to scout this + make the necessary defensive 8 lings. Another tell is an early sentry almost definitely indicates a robo opening.

    In summary, I think the results are clearer than they seem, I would not expect any difference in winrate in ZvT were there to be no pillars inside terrans natural, but the difference in ZvP winrates makes sense.

  4. Hi.

    Long time lurker, first(?) time commenter. I think you’re less able to draw inferences from the data about scouting pillars than this post implies, because of causal assignment issues.

    Specifically, mapmakers (try to) optimize their maps to be perfectly balanced for each of the 3 asymmetric matchups, and map selectors also have some control over which maps get picked in the pool (and usually they select maps that they perceive to be more balanced). Assuming map makers/selectors are perfectly calibrated, we would expect the effects of scouting pillars *to be exactly counterbalanced* by commiserate disadvantages given to Zerg elsewhere on the maps or advantages given to the other two races.

    Now in practice mapmakers are not perfectly calibrated so win rates on any given map may be far away from 50%. But we can’t ex ante predict which direction this miscalibration will be. So the naive empirical win rates on different maps can’t tell you how much of an advantage scouting pillars would be. Instead, for causality you’d want to look at differential win rates in either a) *two otherwise completely identical maps except for the scouting pillars* or b) some clever natural experiment that lets you tease apart causality.

    Or more succinctly in Pearl’s terms P(Zerg win| scouting pillar) =/= P(Zerg win | do scouting pillar).

    I hope this is helpful.

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