What is the Difference Between Point Typical and 90% Confidence Level Mean?
If a sample of parts is stressed and an MTBF value is calculated, this is often referred to a point typical MTBF. If many samples were used and MTBFs were calculated, they would not all be the same. There would be a distribution of MTBF. It has been determined that the MTBF follow a chi squared distribution. Knowing the MTBF values are distributed means that the confidence in any one value can not be that great. To improve the confidence in the point typical MTBF to represent the entire population of MTBFs, a confidence interval is usually calculated. Statistically any distribution can be divided into different regions containing different portions of the population. The 90% confidence interval is the region containing 90% of the distribution (with 5% in each of the 2 tails). The interval can be used as an estimate of what to expect from many other samples (or customer use). The statistical inference made from this calculation is that in 90 out of 100 samples we test, or that the custo
