Strategy games typically place the user in the role of something like a battalion commander. But think about the job of a real battalion commander. He doesn’t sit watching a map and barking orders to each tank on the battlefield. In combat, he has a small number of people he gives direct orders to. A squad leader in the US Army leads about nine soldiers, with the help of two fire team leaders. A platoon leader typically commands 4 squad leaders plus 3-4 other staff. A company commander typically gives orders to four platoon leaders plus staff; a lieutenant colonel might command a battalion of four companies plus staff. And so on, up the chain of command.
This is no accident. It is a fundamental principle of military organization doctrine that a commander can effectively manage only a limited number of subordinates. The number of directly-reporting subordinates that a commander has is known as his span of control. The most efficient span of control is believed to be the same for a platoon leader as for a theatre commander. 19th-century European armies settled on seven as the maximum span of control, and that number hasn’t changed much since.
The rule of thumb in the US military today is that span of control should be from 5 to 7. A supervisor in FEMA is supposed to oversee no more than seven subordinates during a disaster-relief effort. According to US House Report 104-631, the average span of control across the entire US government bureaucracy in 1997 was seven. Not coincidentally, seven is also the number of items that the average person can keep in memory at the same time (Miller 1956). This is why phone numbers within an area code are seven digits.
This rule should apply to strategy game design as well. A player who is controlling more than seven entities can’t effectively supervise any one of them. (A corollary is that, if a turn takes a minute, and a player makes a move more than about once every 10 seconds, that player probably isn’t focused and isn’t getting an opportunity for the kind of deep, strategic thought that is supposed to be the source of enjoyment in a strategy game. Games that require a click per second are arcade games, regardless of their complexity.)
The seven subordinates that a field commander controls often include one or two who don’t actually act in combat, but who merely relay information to the commander. This means that different information displays (such as the city screen and the technology screen in Civ) count towards the seven.
There are complications to this rule. Take chess – each player controls 16 pieces, and must be aware of 16 enemy pieces. That’s why chess is so frustrating to a beginner. But chess experts can do it. Are they breaking the rule of seven?
Well, yes and no. Gaining expertise in chess doesn’t consist of learning to keep track of more and more pieces in your head. It involves learning to break board positions down into separate, familiar structures – a pawn structure, a castled king behind three pawns and a knight, a set of pieces exerting influence on the center squares – in order to bring the number of concepts down to a manageable number (say, oh, seven).
The span of control in the US military is intended to be such that a commander can command and supervise one level down (his subordinates), and keep track of everything happing two levels down (his subordinates’ subordinates). The chess player is responsible for two levels of control: first, choosing the subgoal for each of these structures (e.g., “control the center” or “break up his defensive pawn structure”); second, choosing a move to implement each subgoal, and choosing just one of those moves. Thus, it isn’t keeping track of all the chess pieces, but having to choose every move, that violates the rule of seven. If you want your game to play faster than chess, and still involve strategy, you must observe the rule of seven.