Quote Originally Posted by 'daviddoran',index.php?page=Thread&postID=76111#po st76111
Completely random. Certain items are more likely than others to drop, so you are just experiencing that liklihood. If one item has a 60% chance and another has a 30%, on a given run you are more likely to see the 60% chance one, even if you run it over and over repeatedly.

Anyone who has a "method" of influencing loot is just using voodoo magic. And we all know the only ones who can do Voodoo are Troll Witch Doctors...
While it may seem not-so-random at times, I've played poker with friends thinking the same thing if I get on a rather unlucky streak. A lot of it has to do with cumulative probability. A boss that only drops an specific item 20% of the time will need to be run 10 times to have a 90% total chance of dropping that item. That means that even if you run a boss 10 times, there's a 10% chance of that item not dropping, and that's for an item with a 20% drop rate. It gets much worse for those 5% or 1% drop items!
The simple formula for how many runs (X) to have a total chance (Y) of a specific item drop (Z) is the following:
((1-Z)^X)=1-Y.
For example, you really want a very rare item, so you want a 90% of getting it in a given night (As long as the drop rate is less than 100%, you can NEVER have a 100% of getting it even for an impossible number of runs). Unfortunately the item only has a 5% drop rate on the boss. You'll have to run the boss (1-.1)^x=1-.9 times.
(1-.1)^x=1-.9
.9^x=.1
x=22
22 runs to have a 90% chance of getting a 5% drop item from a boss in at least one of those runs. Grind away!