So what happened in 3rd Edition?
As far as I can tell, everybody misread the rulebook. Here’s what the 3rd Edition Dungeon Master’s Guide had to say about “Encounters and Challenge Ratings” (pg. 100):
A monster’s Challenge Rating (CR) tells you the level of the party for which the monster is a good challenge. A monster of CR 5 is an appropriate challenge for four 5th-level characters. If the characters are higher level than the monster, they get fewer XP because the monster should be easier to defeat. Likewise, if the party level [....] is lower than the monster’s Challenge Rating, the PCs get a greater reward.
And a little later it answered the question “What’s Challenging?” (pg. 101):
Since every game session probably includes many encounters, you don’t want to make every encounter one that taxes the PCs to their limits. They would have to stop the adventure and rest for an extensive period after every fight, and that slows down the game. An encounter with an Encounter Level (EL) equal to the PCs’ level is one that should expend about 20% of their resources — hit points, spells, magic item uses, etc. This means, on average, that after about four encounters of the party’s level the PCs need to rest, heal, and regain their spells. A fifth encounter would probably wipe them out.
And, at that point, everybody apparently stopped reading. Because this was what seeped into the collective wisdom of the gaming community: Every encounter should have an EL equal to the party’s level and the party should have four encounters per day.
I literally can’t understand how this happened, because the very next paragraph read:
The PCs should be able to take on many more encounters lower than their level but fewer encounters with Encounter Levels higher than their party level. As a general rule, if the EL is two lower than the party’s level, the PCs should be able to take on twice as many encounters before having to stop and rest. Two levels below that, and the number of encounters they can cope with doubles again, and so on.
And if that wasn’t clear enough in saying that the PCs should be facing a wide variety of ELs, the very next page had
a chart on it that said 30% of the encounters in an adventure should have an EL lower than the PCs’ level; 50% should have an EL equal to the PCs’ level; 15% should have an EL 1 to 4 higher than the PCs’ level; and 5% should have an EL 5+ higher than the PCs’ level.
But all of that was ignored and the completely erroneous “common wisdom” of “four encounters per day with an EL equal to the party’s level” became the meme of the land.