Effective character level (ECL) is a way to compensate for certain classes having a harder time at low levels in Neverwinter Nights (especially in single player). DnD accomplishes this by counting some classes as a lower level when calculating the experience points for killing monsters, though NWN implements this by instead considering the monsters killed to have a higher challenge rating (CR) based on the class levels of the killer. With the expansion packs, NWN abandoned the use of ECL, and thus it is only used in the original campaign and in modules made with a Toolset that does not have the expansions. For a multiclassed character, the bonus to CR for the monster's killed is the sum of the bonus for each class (by its class level).
Levels Beyond 20
The original campaign did not factor in classes going beyond 20 levels (as there was a level 20 cap when it was released). However, playing this module (or a module made with a Toolset without the expansions) with a character with classes beyond level 20 (see hordes of the underdark expansion) will cause that class to give a CR bonus equal to the entire class level. Thus a Wizard 5/Cleric 5/Sorcerer 30 would cause a CR bonus of 3 + 3 + 30 = 36, and thus would get as much experience points for killing a creature with a challenge rating of 4 as killing one with a challenge rating of 40 (40CR is the maximum for XP calculation).
Pen and paper comparison
In pen and paper D&D, ECL is used to compensate for the traits of the more powerful races. The average drow, for example, has several powerful abilities that make her more powerful than a human, and so would always be considered to be two levels higher than the sum of her class levels. Thus a drow character would gain levels slower than a human, balancing out the racial advantages.
- Some modules have scripted various systems to implement subraces, and some of these include pen-and-paper style ECL. One such system utilizes racial hit dice, but this solution has the drawback of using one of the character's three potential classes for these racial hit dice, limiting characters with racial hit dice to two "real" classes.