I've done over 50 runs (did not record data), and at an eyes glance, I'd get an orb around every 1.33 runs and the occasional double orb run.
While I do not have any data to show for this, I'd like to share my experience in farming this was very positive
Edit: They drop Advanced and Elite orbs
I'd think it might have the possibility of being changed or it's just all coincidental
It's pretty good for advanced orbs. Although elite don't show up that much, I'd rather do d14 missions for those.
Sewer stash is ,according to popular belief, the best place for advanced-elite orbs; with around 1-3 every 1-2 runs, depending on selected difficulty. It is,however, a static, repetitive mission with arguably the hardest battlepod outside of places like danger missions.
Man, quit hoarding all the luck, y'all! The last time I spammed Sewer Stash for orbs, I went 20 runs of the first two depths without a single advanced OR elite. e.e
On sewer stash i got 2 elites on my second run and I never saw another since....
Sewer stash is ,according to popular belief, the best place for advanced-elite orbs; with around 1-3 every 1-2 runs, depending on selected difficulty. It is,however, a static, repetitive mission with arguably the hardest battlepod outside of places like danger missions.
I'm pretty sure you're supposed to be farming only the first depth.
The first depth isn't static (though it is a bit repetitive) and has no battlepods.
EDIT:
Man, quit hoarding all the luck, y'all! The last time I spammed Sewer Stash for orbs, I went 20 runs of the first two depths without a single advanced OR elite. e.e
Sounds like you're playing on normal.
Play elite if you want orbs.
And if you have been, well...
I've seen Sewer Stash recommended many times for farming orbs. I haven't seen any serious justification other than the data set linked in post #1 above, which shows Sewer Stash giving disproportionately many orbs. And that data set is about running the whole mission, not just the first level.
I do the first two depths of Sewer Stash, the one with the battlepod isn't worth it unless you need just a little bit more of heat or something.
Sewer stash just is regular level generation for the first two levels, and both levels are easily speedrun-able.
Lichenous lairs are pretty quick to speedrun, and slimes are also weak to the more overpowered weapons in the game (acheron, grim repeater), but has a few forced arena rooms. Clockwork tunnels don't have many forced arena rooms, but will slow you down with puzzles, but can grant more prize boxes.
Personally, despite my hatred of slimes, I run only the first level of sewer stash as the clockwork tunnels level seems to drag on a bit too long for my taste.
http://wiki.spiralknights.com/Lancer_Knightz_%28Guild%29/Mission_Rarities
They have 15 measurements of orbs for Rise or Fall. That's not as many data as you, but they are carefully curated.
They average 0.2 3-star orbs and 0.07 4-star orbs per run. In other words, they got a total of 3 3-star orbs and 1 4-star orb in 15 runs. So they get 1 orb every 3.75 runs.
But 15 measurements is few enough that the uncertainty is still pretty high. They don't show confidence intervals.
And it's conceivable that the drop rates have changed over time, although the bottom of the page shows that it hasn't for Radiants.