“If the available power grows at exactly the same rate as the absolute difficulty goes up, the relative difficulty will be a flat line, as illustrated in Figure 11.4 (next page). In that case, a level 5 knight would find it exactly as hard to kill a level 5 troll in the middle of the game as a level 1 knight would find it to kill a level 1 troll at the beginning of the game. But relative difficulty should not be a flat line because when you factor in the player’s increasing in-game experience, the perceived difficulty actually goes down—the game gets easier. Aim to increase the absolute difficulty of the challenges somewhat faster than you increase the available power to meet them. The gap between absolute and relative difficulty widens only slowly.”