In a product design class, the prof asked us to draw the distribution of resistors in a bag of, say, 5-ohm resistors, bought from a local electronics shop.
It turned out that a normal distribution centered at 5 ohms was not quite right. It was a normal distribution, but with a deep notch taken out right at 5 ohms. All the resistors that tested to very close tolerance had been bagged separately, and sold at a higher price.
(The context was why you might want to put, say, four 5-ohn resistors in series, rather than just use one nominal 20-ohm resistor.)
I hope your product design class covered error propagation, because one 1% 20 ohm resistor would still be cheaper and more precise than 4 5% 5 ohm resistors.
But four 5%-tolerance 5-ohm resistors could be chosen to add up to almost exactly 20 ohms, but a 20-ohm resistor would never be 20 ohms due to the notch.
In the old days, it might have been cheaper than the more accurate resistors. These days, high precision resistors are relatively inexpensive.
It turned out that a normal distribution centered at 5 ohms was not quite right. It was a normal distribution, but with a deep notch taken out right at 5 ohms. All the resistors that tested to very close tolerance had been bagged separately, and sold at a higher price.
(The context was why you might want to put, say, four 5-ohn resistors in series, rather than just use one nominal 20-ohm resistor.)