Do you earn more credits if you're actively playing tiny death star (elevator, etc) or while the game is closed and off?
I performed the following experiment, in each case counting the credits earned and number of levels that were open and selling items. Note that my elevator is the 10PPM model:
These two tests were performed consecutively. It appears that TDS uses a different model to earn credits while you are actively playing the game vs when it restarts and calculates all the expected earnings.
The difference is staggering, and if your goal is credits you should be playing for brief periods of time infrequently, rather than constantly. You won't be able to make many bux that way, though.
This is from tests conducted after the 1.3 update in early February 2014, and this may change over time, particularly since it appears to be a bug - you should be earning according to your elevator speed in the game as well as out of game. Tested on iOS - I expect it will be the same on other platforms, but it's possible that the timing code is platform specific, so it might be different. Would be worth more testing.
I'm on Android, my data (with the phone closed between writing down the numbers):
At 3:41 my store's stocks were
193, 245, 305
62, 114, 174
31, 84, 144
That's a steady 10 customers per minute. I have the level 4 elevator, which says it has a rating of 14 ppm.
(I need to redo data while open)