Actual Capacitor Tolerances

I'm designing a filter using 0.1uF 10% X7R caps. I've measured a fair number of parts straight off the reel and all seem to measure 5-15% BELOW the nominal value (i've used a couple meters... both pretty decent).

From what I've read, capacitors come off the assembly line with values in a normal distribution... the "middle" 5% parts are cut out and sold for a higher price, so 10% parts actually lie in a bimodal distribution centered around the nominal value...

My question is why haven't I seen any values in the upper part of the distribution??? Have I just not measured enough parts, or can I expect to not see any values ABOVE nominal? (Seems to me capacitor manufactures could make more money selling the upper distribution as more expensive 0.12uF caps, or save material by centering the original distribution slightly below "nominal".)

Any insight is appreciated.

What kind of ,"meter," have you been using? Also if you got parts from the same lot which were manufactured in approximately the same time frame through out the stats. Not saying the measuring device is the issue, it wouldn't through off the distribution.

You are correct WRT the distribution. So when you don't see that, it just means you don't have a representative population, or that's what it should tell you. Depending on how many parts you have you may need hundreds to get some confidence in say a lot of 10,000. Dig up your stat book on sampled standard deviation, it'll tell you our confidence. Oh and now that they have been sorted and removed the upper distribution it's no longer binomial, or random is it.

Are they marked parts from a reputable maunfacturer? Are you measuring them the same way (voltage bias, frequency) they are specified? Do any other valued parts measure within range on your meter?

Check out the bottom of page 1 here to see how the capacitance vaies with these parameters. http://www.niccomp.com/catalog/nmchc.pdf

The capacitors also have a Tolerance rating expressed as a plus-or-minus value either in picofarad's (±pF) for low value capacitors generally less than 100pF or as a percentage (±%) for higher value capacitors generally higher than 100pF. The tolerance value is the extent to which the actual capacitance is allowed to vary from its nominal value and can range anywhere from -20% to +80%. Thus a 100µF capacitor with a ±20% tolerance could legitimately vary from 80µF to 120µF and still remain within tolerance.

Capacitors are rated according to how near to their actual values they are compared to the rated nominal capacitance with coloured bands or letters used to indicated their actual tolerance. The most common tolerance variation for capacitors is 5% or 10% but some plastic capacitors are rated as low as ±1%.

Check out the the link http://actpcb.com/capacitor-calculator where you may got enough query for capacitor tolerance.