8.
Assessing Product Reliability
8.2. Assumptions/Prerequisites


Create a probability plot and if the points line up approximately on a straight line, the assumed model is a reasonable fit  Graphical plots of reliability
data are quick, useful visual tests of whether a particular model is consistent
with the observed data. The basic idea behind virtually all graphical plotting
techniques is the following:
Points calculated from the data are plotted on a loglog scale and, as long as they line up approximately on a straight line, the analyst can conclude that the data are consistent with the assumed model.If the reliability data consist of (possibly multicensored) failure data from a non repairable population (or a repairable population for which only time to the first failure is considered) then the models are life distribution models such as the exponential, Weibull or lognormal. If the data consist of repair times for a repairable system, then the model might be the NHPP Power Law and the plot would be a Duane Plot. The kinds of plots we will consider for failure data from nonrepairable populations are: For repairable populations we have
Note: Many of the plots discussed in this section can also be used to obtain quick estimates of model parameters. This will be covered in later sections. While there may be other, more accurate ways of estimating parameters, simple graphical estimates can be very handy, especially when other techniques require software programs that are not readily available. 