Short version: Some capacitors (and presumably some other components) are sold with imbalanced/asymmetrical tolerances. Why?
Many ceramic capacitors are marked with, for example, +80% -20% tolerance or something similarly imbalanced.
For example, let's say that I have a capacitor with the (admittedly contrived) value of 17pF and a tolerance of +80%, -20%.
(Please ignore abuse of significant figures.)
- Maximum value: 17pF * (1 + 80%) = 17pF * 1.8 ≈ 30.6pF
- Minimum value: 17pF * (1 - 20%) = 17pF * 0.8 ≈ 13.6pF
- Mean value: (30.6pF + 13.6pF) / 2 ≈ 22.1pF
- Tolerance above mean: (30.6pF - 22.1pF)/22.1pF ≈ +38.5%
- Tolerance below mean: (13.6pF - 22.1pF)/22.1pF ≈ -38.5%
It would be fair to say that this supposedly "17pF" capacitor is virtually identical to a 22pF capacitor with ±40% tolerance.
By a similar process, a 10000pF +80% -20% capacitor (a real value from a catalog, not contrived) should be the same as a 13000pF around ±40%.
So, if I say I want a component of a given value, why am I being sold something that's quite a bit more likely to overshoot than undershoot this value? Is this imbalance useful to anyone?