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Better Game Design through Data Mining Why Not To Mine Data We have glossed over the general process. Let's now step back and consider a healthy scope for data mining. Data mining provides answers that other methods of evolutionary game design cannot. However, it is not a panacea. Data mining takes numbers, processes them, and makes new numbers. These numbers cannot tell you how each player feels. The player may be misinformed or biased about the balance of the game, but she is always right about how she feels. Some players' feelings may be immature, and some players may have contradictory responses. Yet the paradox is that they are all right. Every player's emotional response is valid. The data also does a poor job of revealing how players feel about each game asset. It does not indicate which asset has beautiful modeling, expressive animation, or a compelling story.
A healthy scope excludes preemptive or preventive data mining, which attempts to identify and prevent cheating, harassment, or sabotage. This equates to profiling and an invasion of privacy. Besides being unethical, preemptive data mining is disastrous. Data mining cannot establish cooperation or culpability. Not only is it prone to random error and false positives, but also it creates a new source of player harassment. This source of harassment is hard to discover, impossible to eliminate, and much more costly: Harassment by your own staff upon your customers. Data mining is also called knowledge discovery. While you can mine knowledge from data, you cannot mine wisdom. You have to prioritize results and decide which game imbalances should be left alone. Data mining automates a process within your overall evolutionary design cycle. It amplifies an efficient design process and multiplies the problems in a poor process. Four practical uses Now that you have seen the general process, let's apply it to some common MMOG problems. Here are four practical applications of data mining:
Balance The Economy Each game asset that passes hands between players is a commodity or currency. These tradable game assets define the game's economy. The commodities and currencies need not be limited to money and property. For example, in Nexon's Dark Ages, I designed and implemented a labor currency, a political currency, and a religious currency.
Be careful when measuring individual character gains and losses. Account for transactions that exchange one commodity or currency for another. For example, a character could have less money after one week but have more wealth. He may have exchanged his money for other commodities of greater value. Track the game's macro-economic indicators. See if the supply of currency is increasing or decreasing. Like a real-world money supply, this tells you about the inflation rate of the currency. Measure key performance indicators and generate hypotheses of how to improve game balance. One simple balancing technique you can use is to change the price of a game asset. Players are more receptive to price changes than they are to other attribute changes. For example, in 2002 when Stewart Steel noticed low admittance rate for wizards in Nexon's Nexus: The Kingdom of the Winds, he increased the rate by increasing the starting items of that class. In effect, this increased the price that an NPC paid the player for choosing the wizard career.
After testing the hypothesis, repeat the cycle each month. Each modification, although seemingly insignificant, can have a huge ripple effect on the rest of the economy. In the same example, if there were a higher starting value but a poor prospectus for the career of a wizard, then retention rate among wizards might drop. While
balancing the strategies, such as player classes in a fantasy setting,
ensure that each strategy remains unique. Keep the clusters in strategic
space from converging. Let's return to the original example. The
low-performing, high-level fighters have several unique and shared
assets. When adding a new asset to balance their performance, it
might be better not to give a fighter "Poison Tolerance."
If the Priest class has an ability to cure poison, then this would
be redundant. It would reduce the group's demand for Priests and
begin to merge the two classes. Instead it might be better to provide
"Sword Mastery" if no other class has this kind of ability.
This controls the supply of assets so that each cluster of assets
retains its unique niche in the game.
Catch Cheaters A cheater in a MMOG does not just cheat himself. He performs an injustice to all honest players. Cheating short-circuits gameplay, so it achieves exceptionally high performance. Players adopt high performance strategies, whether intended by the designers or not. Cheating also penalizes the relative performance of all non-cheaters. If not corrected quickly, cheating will spread like wildfire. In a matter of weeks or even days a cheat can flood the game's economy. These techniques can help catch cheating before it ruins the economy. Start at the table that preprocessing generated. This lists each character ID and their performance. Sort the list by the performance column. Now, at the top of the list is the most suspicious character ID. Investigate his exceptional performance.
Let's sort the example table by the EPH column. The character at the top is the most suspicious. Even though he has a lower total experience gain, he has a higher rate, since he accumulated the experience during fewer hours. Investigate the logs to discover how he performed so well. The answer will enlighten you as to how players use and abuse your game. The answer does not indicate the player's intention. The player may have been using a legitimate feature of the game. In fact, a player may argue that unless he modified the software, all of his behavior is a legitimate use. He played the game as it was given to him. Regardless of the motive, deleting a cheater cannot solve the problem. System imbalances breed cheaters, so the design itself can prevent cheating. Cut Production Costs Each game asset took some amount of programming, art, design, testing, and customer service to develop and maintain. Yet some of these classes, items, monsters, quests, skills, zones, and other objects in your game are being wasted. This lowers developer morale. Game assets with low performance have a return-on-investment value that approaches zero. Players make decisions and optimize their decisions over time, and communication among players accelerates the migration to an optimal strategy. They quickly adopt the highest performing game assets available, and discard low-performance assets. In terms of competition, these assets are liabilities, so they become obsolete. For an obvious example, if there are two nearly equivalent weapons, except one has a higher damage rate, the other weapon is obsolete. In a game, this kind of decision, between obsolete assets and newer assets creates "fat"; there is some fraction of your game that might as well not exist, because no one uses it. Imagine having to break this news to an artist: "Thank you for the long-nights you spent making this new graveyard that we specified, but no one hunts there. Sorry about that."
It does not have to be this way. A wise patch can put the assets back into the players' list of options. Recycle the artists, programmers, and testers' hard work as much as possible. Create new, well-balanced instances. Measure and prove their balance in terms of performance. Do not change the values of existing instances. Let them remain as they are. Recycle the art with modest modifications so those man-hours are not lost. But only recycle obsolete assets. Players do not tolerate recycling of assets that they do not consider obsolete. They demand fresh assets. Increase Customer Renewal If the player cannot or does not realize how to improve his performance with the choices he has already made, he is doomed. For example, if all high-level fighters perform worse than average high-level characters, all fighters are doomed. The players' sense of doom will become the developer's death knell unless you act fast.
When a player suffers from poor performance in a single-player game, he suffers alone. But in a massive multiplayer game, his whole team suffers. Unfortunately, a good choice for the team to increase their performance is to exclude low-performers. When low-performance is not the player's fault, this breeds frustration. Suppose a group of players can increase its EPH 20% by excluding low-performance players. Sadly, many groups will. Suppose the excluded low-performer is unable to alter his EPH liability. Like an endangered species that is unfit to hunt and unable to evolve, this set of players becomes extinct. The character will not only become extinct from the playscape, but the player's motivation to play will become extinct, too. If she will not play, eventually she will not pay, either. To prevent this, balance the strategies. Do not edit existing instances of game assets. This will upset other players using other strategies. They will perceive the correction as an injustice, an act of favoritism. Instead of creating a perceived injustice, add new assets. If your game is commercial, improve player performance instead of worsening it. If some asset is too good, but players love it, let it be. Only when the asset would cause long-term customer losses should it be removed, because removing or degrading an asset decreases customers' good faith. There are few things that say, "I do not want your money, go away" as quickly as removing a beloved feature in the game. Players paid money in advance and continue to pay a subscription fee each month for a reason. They expect the game to improve each month. Their criterion is simple. The game should improve for their character personally and for the special interest group that their character belongs to. A major error's existence costs more than this loss of good faith. In these uncomfortable cases, prove to players you care by negotiation and diplomacy. Imagine a worst-case scenario from the most extreme player's perspective: This morning your paycheck was suddenly slashed 50%. Your brand of car drove half as fast, required repairs twice as often, and costs twice as much. Because the "gods" said so. Some customers take the game just as seriously. Go Forth And Mine! We have only touched the tip of the iceberg of data mining and game design. Both are elaborate and exciting fields for research, experimentation, and application. For years, we game designers have wanted systematic and scientific tools, and I hope that data mining is one such tool to help improve your game's design. If you have questions, comments, or would like to discuss this topic in detail please contact me at kennerly@sfsu.edu. Acknowledgements I could not have written this article without the support of each of Nexon's employees and players. They encouraged my experiments. Further Reading Han, Jiawei and Micheline Kamber. Data Mining: Concepts and Techniques. Morgan Kaufmann Publishers: San Francisco, 2001.
Hand, David, Heikki Mannila, and Padhraic Smyth. Principles of Data Mining. MIT Press: Cambridge, 2001.
Electronic
Privacy Information Center. "Total
Information Awareness (TIA)" April 20, 2003
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