The curious case of the 7% resistors

Ever have an application where you needed resistors with accurate values, but couldn’t find any in your pile of cheap ones that had exactly the value was marked on them? Howard Johnson of the Electronics Design, Strategy, News blog has a surprising explanation for the phenomenon in his article, the 7% solution:

The drawing complete, Martin said, “A 10% carbon-composition resistor is made in a somewhat slipshod manner. The manufacturer tries to get it right, but some of the variables are just too difficult to control. They make up a batch, test them all, and then throw away the bad ones. What’s left is a distribution of values truncated on either side at the ±10% limits. The other main feature of the distribution is the big gap-toothed section in the middle. That’s where they pulled out all the good parts and sold them at a higher price with a ±5% tolerance. How else do you think they make 5% resistors?”

I’ve never taken the time to measure a batch of resistors to verify that this is true, however it certainly sounds plausible. Anyone here have some first hand experience with this? It seems like it would be a great exercise to go through in a practical electronics class. [via mightyohm]