Mentzer later adds on to the above in his Expert book, pg 25: "The DM may choose a number within a given die range rather than roll for the amount of damage, number appearing, etc. This may be necessary to allow for a more enjoyable game; heavy damage early in the game may spoil some of the fun."
Mentzer also included the following section in his Basic book, pg 16: "Whenever two or more events could occur and a decision is needed, or whenever a variety of results is possible, dice may be used to randomly select a result. Experienced Dungeon Masters may select results instead of rolling dice.
EXAMPLE: A character with 3 hit points is hit by a monster with a normal sword (damage 1-8). Death could easily result - through no fault of the player's, merely a random result of the Hit roll. To keep the character alive as long as possible, the DM rolls for damage - but ignores the result, and announces that 2 points of damage were done. The character retreats and the game continues.
...
The DM may select any number given in the range for Number Appearing, Damage, and so forth, depending on the situation."