Being an economist at heart, I have attempted to condense the process of educational game evaluation into a simple conceptual equation:
tT * pP = Learning
where

 T = Time spent learning
 t = The increase in time spent learning, as a result of higher engagement brought about by using the game. (With no game, t = 1. A game that doubles time spent learning would have t=2. Etc.)
 P = Productivity of learning. (Learning / Time)
 p = The decrease in productivity of learning per unit of time, as a result of the extraneous game mechanics that are added to a game, such as storytelling, character development, score reporting, smack talking, etc. (With no game, p = 1. A game that is purely “fun” and completely eliminates all learning benefits would have p=0.)
The goal of educational games, of course, is to ensure that the implementation of the game increases learning by enticing the student to spend more time on the activity, without losing so much time in productivity. In equation terms, we can say:
t * p = Δ Learning,
or
If (t * p) > 1 ,The game is worthwhile
Although it is obviously very difficult to measure exactly what t and p actually equal, the important point is to keep these concepts in mind when evaluating games. For example, the classic game Math Blaster is very good at increasing students’ motivation to spend more time drilling their core arithmetic skills (i.e. it has a high t), while almost constantly keeping the student engaged in math problems (i.e. it has a p barely less than 1). Perhaps 95% of all seconds spent playing Math Blaster are spent calculating a figure in one’s head and then inputting the answer. The calculation is a nobrainer; Math Blaster is a great tool for early math education.
In contrast, the game Where in the World is Carmen Sandiego? is a much tougher game to evaluate. Sure, a student is likely to spend a much larger amount of time playing the game than she would have otherwise spent studying Geography (i.e. the game has a very high t). But what percentage of the time spent playing Carmen Sandiego is spent actually learning? Most of the game is really devoted toward the storyline, character development, and mystery, with Geography learning coming only tangentially (and often without enough repetition for a concept to truly stick). Carmen Sandiego has a very low p. It is less obvious that class time should be devoted toward playing the game.
Of course, every class is different, and every student has different needs for being motivated. (Some “problem students” might never study unless it is a game, meaning the game has a much higher t factor for them than for other students.) But educators should be careful to weigh “fun” against productivity when choosing whether to use a game in their classroom.
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