While I was at the bodega today, buying Bubble Yum and popcorn, I counted my $1.34 in change and thought about order checking and error proofing. Wow, do I sound boring. But listen.
Counting your change is essentially checking a process. It's the same as a check process for order taking, a check process for picking, and so on. It's checking the change-making process. The clerk tells you the price. You hand them the dough. The clerk tells you how much the change is. They put it in your hand. You count it. Is the amount correct? If it's correct you say Thanks and leave. If it is incorrect you say Whoa immediately and get it fixed on the spot. They key thing here is that the check is immediate.
Imagine another scenario. Instead of immediately checking your change you saved it to the end of the day. You did that with every transaction all day long. At the end of the day you reviewed all the transactions to check if they were correct. How much extra work would it take? How much paperwork and extra record keeping would you have to create? How much extra work would it take to fix the correction, to go back to the clerk and convince them that there was an error?
Now imagine you saved it till the end of the week. Or the end of the month.
This is essentially the concept behind building quality into a process versus confirming quality at the end. When you build quality in, it's not that you don't check. It's that you check immediately. Reducing the time gap between the action and the check has the largest effect on quality. It also has a tremendous effect on cost since it usually requires a lot less paperwork and bureaucracy.