Hazard and Cum Hazard Plotting

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8. Assessing Product Reliability 8.2. Assumptions/Prerequisites 8.2.2. How do you plot reliability data? Hazard and cum hazard plotting
Another kind of plotting, called Cum Hazard Plotting, has the same purpose as probability plotting Just commercial probability paper is available for most life distribution models for probability plotting of reliability data, there are also special Cum Hazard Plotting papers available for many life distribution models. These papers plot estimates for the Cum Hazard H(ti)vs the time ti of the i-th failure. As with probability plots, the plotting positions are calculated independently of the model or paper used and a reasonable straight-line fit to the points confirms that the chosen model and the data are consistent. Advantages of Cum Hazard Plotting 1. It is much easier to calculate plotting positions for multicensored data using cum hazard plotting techniques. 2. Linear graph paper can be used for exponential data and log-log paper can be used for Weibull data. Disadvantages of Cum Hazard Plotting 1. Commercial Cum Hazard paper may be difficult to find. 2. It is less intuitively clear just what is being plotted. Cum percent failed (i.e., probability plots) is meaningful and the resulting straight-line fit can be used to read off times when desired percents of the population will have failed. Percent cumulative hazard increases beyond 100% and is harder to interpret. 3. Normal cum hazard plotting techniques require exact times of failure and running times. 4. With computers to calculate the K-M estimates for probability plotting, the main advantage of cum hazard plotting goes away. Since probability plots are generally more useful, we will only give a brief description of hazard plotting. How to Make Cum Hazard Plots 1. Order the failure times and running times for each of the n units on test in ascending order...
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