ABSTRACT
The Excel spreadsheet used to generate the Poisson tables has, in general, been instructed to show only two decimal places. Meanwhile, the Excel program carries eight decimal places internally, even when the program is instructed to show fewer than eight places. Reliability figures are based on failure rates for each of the individual components in an assembly. When added together, the sum of the individual failure rates becomes the failure rate for the entire assembly. In practice, for Poisson calculations, 60% confidence level is used instead of the 50% associated with averages in normal distribution calculations. 60% is used, because of the skewness of most Poisson distributions. This unbalance of the Poisson distribution causes the Mean Time Before Failure (MTBF) to occur at 37% of the time span of the distribution. In actual practice, most calculated MTBFs are rounded up to the nearest hundred.
