XP, Leveling, & Measuring Game Difficulty

My method for quantifying Labyrinth Lord/B/X adventure difficulty includes the following designations by average monster Hit Dice range:

Low-Level:  1-6
Mid-Level:  7-14
High-Level:  15-20

This designation makes sense when you look at the trend in XP requirements for leveling up.  The vertical bar graphs below shows the XP required for each level in Moldvay D&D.  Below that is a line graph showing the average for all classes of the XP required for each level advancement.

Graph 1:  Each bar represents the XP needed for each level (x-axis) up to level 8.

Graph 2:  Each bar represents the XP needed for each level (x-axis) up to level 20.
 

Graph 3:  The line represents the average for all classes of the XP needed for leveling up.

It appears that all classes progress throught the first seven or eight levels in a relatively similar numerical pattern; it is close to doubling each time.  It isn't until human characters progress beyond level 8 that the XP required increases at much greater variability.  Once level 14 is reached, the increments remain consistent on average (see graph 4 below).  This supports my ranges for the three categories above.  As you can see from the graph below, once demi-humans start dropping out of the picture, there is a range (levels 8-14) of increased variability in XP needed for advancement.

Graph 4:  The line represents the change in average XP needed for leveling up.

What does that mean?  For one thing, either a DM boosts the available XP for mid to high-level parties, or level advancement slows down considerably past level 14.  It also means that mid to high-level play for demi-humans is significantly different.

On a side note, I was also surprised to observe that it was clerics who progressed far faster than magic-users.  I had always assumed (without really looking at the numbers) that it would be fighters (see graph 2).  It makes me wonder if there is really a need for differences in level advancement at all.

Graph 5:  Comparing the XP needed for leveling up between Clerics and Magic-Users.

An idea I've been toying with is to use cumulative monster XP and compare it to graph 3 (above) to determine adventure difficulty.  Such a method creates a singular difficulty number instead of range.  This method also accounds for number encountered and special monster characteristics, something my older method did not.  A major drawback of this method is the somewhat cumbersome and time consuming calculations involved.  If the monsters are arrayed in a spreadsheet, however, either method would be equally simple.  Perhaps I should start including such tables in my future publications.