Question: What are the main principles behind Lean?
Mary Poppendieck: The main principles behind Lean were articulated by Taiichi Ohno, the person at Toyota who invented the Toyota Production System. The first principle would be the idea of Flow (or Low Inventory, or Just-in-Time).
The second one is what I would have to call "expose problems" or "no workarounds". The idea is that you have flow and you have low inventory: it's like you have a boat on the water and your boat is sailing above these big rocks, which are problems. This is your inventory level here. If you lower your inventory level, at some point you're going to run into a rock rock and your boat is going to come down and bump into this rock. If you don't get rid of these rocks when you lower your inventory you're going to bump into rocks, and crash and burn.
The first thing you're doing is lower your inventory to expose those problems so that you can get rid of them. If you don't expose problems and stop and fix your problems, then lowering your inventory is just going to crash your boat. You have to do two things: you have flow but you also have no tolerance for abnormality. You just don't allow defects into your system; you don't allow things to go wrong. When something wrong happens you stop, you figure out what is causing it, you fix it rather than continuing on and just ignoring it or working around it. These are the two basic principles.
Source: "Interview: Mary and Tom Poppendieck on using Lean for Competitive Advantage" on Infoq.com