& CONCLUSIONS Calculations of Instantaneous MTBF have gotten sloppy since J. T. Duane [1] and E. O. Codier [2] published their papers on the subject 43 years ago. Codier presented the following notes on plotting the line through the points in his definitive description of Duane's Reliability Growth calculation methods that he presented at the 1968 Annual Symposium on Reliability:• The latter points, having more information content, must be given more weight than earlier points and • The normal curve fitting procedure of drawing the line through the "center of gravity" of all the points should not be used.• Unless the data is exceptionally noisy, the best procedure is to start the line on the last data point and seek the region of highest density of points to the left of it. These principals for estimating MTBF from nonhomogeneous data are not being followed and the result is a less than accurate estimation of current (aka, instantaneous) MTBF.This paper describes a spreadsheet method for following Duane and Codier's original recommendations that will avoid the errors in judgment that are common when "eyeballing" the growth line or the greater errors that result from using the allto-available "trend line" method. The proposed method automatically weighs the cumulative points (each successive point weighing more), calculates their center of gravity, draws a line from the center-of-gravity point through the last point, and calculates an instantaneous MTBF. Calculations have been made using these methods as well as the common method of drawing a line through those same points using a spreadsheet trend line (aka, least squares fit) and been applied to a multi-year field reliability program. The calculation of instantaneous MTBF (MTBFi) for each of these methods was compared with a moving average centered about successive points over the life of the program and as a control measure of MTBF.We will show that if we adhere to the recommendations of the original papers by Duane and Codier and develop a method to weigh the latter points more, and draw the line through those weighted points and the last point, that significantly better estimates of instantaneous MTBF result. The error in calculation of MTBFi using an automated version of Duane and Codier's recommendations was reduced by 34% compared with a non-weighted trend-line calculation.
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