Stat Tools

Helpful statistical formulas.

An MRR to MTBF calculator: Use this to convert between a monthly return rate and mean time between failures. Be sure to realize this is only valid for the flat part of the bathtub curve!

Failure rate sample inspection sizer: When you have a known occurrence rate of a defect in a widget, it's surprising to see how large your inspection sample needs to be in order to give you good confidence that your population is free of that defect. This uses the binomial statistic, but also gives you a rule of thumb to quickly get you close to the right answer.

ALT Sample Sizer: In order to assess your predicted MTBF when using Accelerated Life Testing, the number of units that are subjected to the test and the allowed failure rate are used for the statistical hypothesis test. This uses a pretty good approximation of the Chi Squared statistic to give you that MTBF.

Acceleration Factor (Norris-Landzberg): Solder joint failures are a common failure mode for electronics. Using the Modified Coffin-Manson equation with specific constants (Apparent Activation Energy and Strain and Frequency exponents), this model is widely used to calculate a multiplication factor that describes how long a harsher Lab test needs to run in order to simulate the expected Field usage effects on solder joint integrity. The empirically-derived default constants are widely used for common solder alloys, but can be adjusted as needed to more closely match observed age-related failure phenomenon.