The state of ADVANCED DUNGEONS & DRAGONS is such that new and different creatures are being devised regularly and often, so that by-and-large, the only monsters which are included are those in MONSTER MANUAL... It is quite possible that at some future time a new edition of this work will be updated to include all of the recognized AD&D monsters. You may do this for yourself now - and include your own favorite creatures at the same time - by finding the experience points value of such monsters and equating them to those already found herein. Determine the frequency (common, uncommon, etc.) of the new creatures, and then include them on the appropriate tables by adding them after comparable monsters already shown. Be careful not to upset the probability balance. [AD&D 1E DMG, 174]
While later AD&D products include the Level & XP in each monster listing (Fiend Folio, Monster Manual II, etc.), this wasn't done in the earlier (pre-DMG) Monster Manual. Instead, for the official Level of the core monsters you have to search through the Random Monster Tables in the DMG (Appendix C), and for the exact XP award you have to separately inspect the Alphabetical Monster Listing (Appendix E). Comparing those two different appendices, here's the point that I want to make today, which goes unstated in the DMG text above:
AD&D DMG monster level categorizations are keyed only to base XP value, and do not include any bonuses for hit points.
Recall that whereas OD&D and the Basic line (Moldvay, etc.) have a simple, single XP value for every monster of a particular type, in the AD&D DMG Gygax complicated things by adding a per-hit-point bonus addition to that. (Back in the day, it took me lots of time to compute XP awards after adventures because you'd have to separately compute every individual monster based on its unique hit point score. Nowadays you could use a computer spreadsheet, etc. -- but still one of the many issues where OD&D wins out over AD&D on usability.) Traditionally when I saw this section in the DMG I assumed that it took into account total XP value, or at least including average hit points for the monster type. Looking closely today at the book examples I see that's not the case -- granted the change in AD&D, it's conceivable that Appendix C here was written before the inclusion of per-hit-point bonuses, such that no such distinction existed as it was being written.
Here are some case examples (flipping between Appendix C for level tables and Appendix E for hit dice and official experience valuations):
- Level I (probably the easiest case to analyze) is said to be anything "up to 20 X.P.". But note how many monsters on the Level I table actually have a base value of exactly 20 XP, such that even the 1st hit point will obviously make them greater than that -- giant ant, badger, fire beetle, hobgoblin, zombie. The manes demon (HD 1, XP 18+1/hp) is also obviously above the mark with average hit points.
- Level II is indicated as "21-50 X.P.", but likewise includes the giant toad (XP 50+3/hp) and troglodyte (HD 2, XP 36+2/hp; so average XP 54).
- Level III is meant to include anything from "51-150 X.P.", but it includes the ogre (HD 4+1, XP 90+5/hp, average 185) and boring beetle (HD 5, XP 90+5/hp, average 203).
- For an example at the higher end, you can look at Level VIII ("3,001-5,500 X.P.") and spot the purple worm monster (HD 15, XP 4900+20/hp, average 6,250). If this was meant to include total hit points, then it would only fit for the very smallest of purple worms -- up to 2 hp per die, something which is statistically very unlikely. Thus it serves as further evidence that the writer of those tables was looking only at base XP value (or even that only such existed at the time of its writing).