By Shevek18

Hearthstone players might be the most superstitious of any e-sport.

Routinely, top level competitive players say things like, “I won’t Imp-losion because I have bad RNG today” or, “I’m really good at drawing Wild Growth.” Smart players know that these expressions are just jokes, but magical thinking about randomness finds ways to creep into almost everyone’s mindset.

RNG, standing for Random Number Generator, has come to be a blanket term for all the diverse effects in Hearthstone with random elements. We blame RNG when we lose, we pray to RNGesus before tournaments games. Many players hate RNG so much that they wish Hearthstone would get rid of it entirely; others dismiss the role of skill in competitive Hearthstone, claiming that the game amounts to a glorified coin flip.

At the same time, RNG is one of the keys to Hearthstone’s mass appeal. Thanks to RNG, Hearthstone is a game where weaker players have a chance to get lucky and win against a stronger player. The more games played, the more stronger players will dominate, of course, but by that time the weaker players are already addicted.

RNG effects also give Hearthstone cards a “wow!” factor. Unstable Portal is mindblowing to new players, because it could never, ever be printed in a physical game. Losing to Mekgineer Thermaplugg off Unstable Portal might be frustrating in the finals of a tournament, but at rank 10 on ladder it makes for a hilarious screenshot.

Randomness can win games that were all but lost and lose games that were all but won. This makes it psychologically difficult, because we all feel as if we ought to win every game, so every loss seems a cosmic injustice. Thinking properly about the strategic value of RNG requires more realistically evaluating our chances of winning. We want to avoid randomly losing games that should be wins, but we also want to maximize our chances to randomly win games that should be losses. That means, in short: play risky when behind, but when ahead, play safe.

Always ask yourself, “Is my chance of a winning RNG outcome higher than my chance to win the game with a different play?” That means taking into account whether you are overall favored or disfavored in the matchup, and how fast you need to play each matchup. It also means keeping track of what cards you and your opponent need to draw to win. What are your opponent’s chances to draw into a win on the next turn? Are you more likely to win if you roll the dice for the win right now, or hope to survive a turn and draw into the win?

Let’s keep this approach in mind while examining some specific types of RNG effects.


Random Damage RNG

Targeted effects with a random amount of damage are probably the easiest RNG effects to think about. In most situations, the correct play is to assume that the effect will deal its average damage. So play Imp-losion assuming that it will deal 3 damage, which happens 2 out of 3 times. Crackle does an average of 4.5 damage, so it requires some judgment about how conservative to be. In an average game, you probably have about a 50% chance to win, so you don’t lose anything by assuming Crackle will do 5 damage. With both spells, if you are behind enough that your chance of winning is lower than your chance of rolling high damage, you can play hoping to steal a win by getting lucky.


Random Target RNG

There are a number of different cards in the game with effects that target a random minion. These include:

  • Dealing damage (Ragnaros the Firelord, Flamecannon, Bomb Lobber)

  • Stealing a minion (Mind Control Tech, Sylvanas Windrunner)

  • Buffs (Coghammer, Powermace, Dark Cultist)

  • Destroying a minion (Deadly Shot, Stampeding Kodo)

  • Destroying everything except one minion (Brawl)

This type of RNG is arguably the most dangerous from a game design perspective, because these type of all-or-nothing effects have the potential to swing otherwise close games completely to one side or the other. There is no average or expected outcome here. That makes it simple to think about, though it can be hard to avoid tilt.

This situation is the purest application of the “play safe when ahead, risky when behind” principle. Enumerate the possible outcomes of the RNG effect and compare your chances of a favorable outcome to your chances of winning with an alternate (safer) play. If your chance of winning with a safe play isn’t better than the chance if you go for the RNG play, it might be worth it to try your luck. In many situations, you can increase your chance of a favorable outcome by trading in advance, for example to minimize your chance of Ragnaros hitting a 1/1, or ensuring that Coghammer buffs a large minion.

Similar logic pertains to cards that have a random effect, such as Enhance-o Mechano and Tinkmaster Overspark.


Many Random Pings

The most complicated type of RNG to think about in Hearthstone is probably effects that cause a series of randomly targeted 1-damage pings. This includes Mad Bomber/Madder Bomber, Avenging Wrath, and Knife Juggler + summoning many minions. Figuring out the exact probabilities on boards with more than a couple minions, for most players, can require a pencil & paper, and take more time than the rope will allow.

We can estimate the probable outcome quickly, however, by observing that many random pings will tend to spread out evenly. Divide the total damage by the number of potential targets (including face), and this is the likely damage that each target will take. If many of the targets have lower health than that, the tougher targets will probably take slightly more damage, since dead minions cannot take additional damage.


Shaman Specific RNG

Lightning Storm is unique in doing a random amount of damage as Area of Effect without being a series of 1-damage pings. It requires slightly different thinking than Crackle or Imp-losion, since, while the odds are 50/50 of rolling high on any specific minion, each specific combination of rolls — including rolling high on everything — is equally unlikely. So, if you only need to roll high on one specific minion, you have a 50% chance of a favorable outcome. The more minions you need to roll high on, the lower your chances get. If you only need to roll high on some minions, but you don’t care which, your odds improve over 50%, as on average you will roll high on half of the targets, and outcomes that are average or better count as favorable.

The design of Lightning Storm therefore fits in well with the Shaman playstyle of always having some type of board presence, since the spell performs best when you can trade to 'clean up', taking advantage of a variety of possible outcomes. It is important to keep in mind which situations give a good chance of a favorable outcome, and which give a poor chance, to avoid the trap of feeling that every low roll is robbing you of a win.

The Shaman hero power is often criticized for its RNG. Blizzard even made the alternate Shaman hero power from Justicar Trueheart summon the normal totems, but without the RNG, instead of a direct power level buff like the other classes. In my opinion, the RNG of the Shaman hero power is a feature, not a bug.

Consider our principle of decreasing variance when ahead, increasing it when behind. The Shaman hero power does exactly that, because it does not allow summoning more than one of the same totem. When a Shaman is ahead, she tends to have a few totems stick to the board, so when she uses the hero power, the RNG is decreased. On the other hand, when a Shaman is behind, she tends not to have any tokens stick to the board, so, while she may get frustrated that she didn’t summon the taunt totem or spellpower totem that she needed right at that moment, the overall increased RNG is exactly what she wants to maximize her chance to steal a game she was already losing.


Card Creation RNG

The craziest RNG effects in Hearthstone come from cards like Unstable Portal and Piloted Shredder, which create cards at random and put them into the game. Ysera, Bane of Doom, Nefarian, and Recombobulator also fit into this category. Card draw RNG effects like Mad Scientist and Deathlord are similar. When thinking about these effects, it is helpful to have a sense of the average outcome, as well as an awareness of potential extreme outcomes that could win or lose the game. For instance, Piloted Shredder on average drops something slightly better than a 2/2. It has the potential to drop Annoy-o-tron, Doomsayer, or Unstable Ghoul (among others), which might be game ending.

With smaller sets of possible outcomes, like Mad Scientist or Ysera, it is relatively easy to keep track of each possibility individually, and judge, for instance, when to play for the 20% chance of getting Ysera Awakens. With a card like Unstable Portal, the best one could feasibly do would be to learn the probabilities of certain important types of outcomes, like minions that can deal at least 2 damage to face, or minions with taunt.


Coping with RNG

The rules of thumb I have described for thinking about RNG effects may seem pretty obvious, but their importance goes beyond planning optimal plays. By consciously thinking about your chances to win the game in comparison to your chances to get favorable RNG in a particular situation, you will avoid a major cause of tilt. You will force yourself to recognize that many of your “RNG losses” were actually lost when you got behind early, and the RNG would have given you an outside shot at stealing a win. Even when you lose those games, you can congratulate yourself on taking appropriately calculated risks. (To reach an even higher level of Zen, you can even notice when your opponent manages to steal wins by taking risks!) You can also recognize better when a win streak represents extremely favorable RNG, rather than an extremely good deck.

One of my personal pet peeves is the myth that control decks in Hearthstone are somehow more consistent for a skilled player than aggro decks. Apart from having no basis in observed win rates, this belief is based in a flawed way of thinking about RNG. A 60% chance to win a game is a 60% chance whether it is spread out over the next 9 turns of card draws or decided right here, right now by a virtual dice roll. The skill in Hearthstone is not minimizing the effect of RNG, but understanding how to take advantage of RNG to maximize your chance to win. Sometimes you are better off trusting Ragnaros than the top of your deck.

Shevek streams Sun, Mon, Wed, and Thu evening (Pacific time) at
You can follow him on Twitter @Shevvek.

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