How About Weibull? |

Quality Digest250 day(s) ago

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How About Weibull? |

Quality Digest250 day(s) ago

The term Weibull in some ways has become a synonym for reliability Weibull analysis life data or reliability analysis The Weibull distribution has the capability to describe a changing failure rate which is lacking when using just mean time between failures MTBF Yet is it suitable to use Weibull as a metric ADVERTISEMENT What to use instead of MTBF Use reliability the probability of successful operation over a defined duration This typically includes a defined environment as well Its the definition of reliability as we use it in reliability engineering Instead of saying We want a 50 000-hour MTBF for the new system say instead We want 98 percent to survive two years of use without failure Be specific and include as many couplets of probability and duration as is necessary and useful for your situation For example you may want to add 995 percent survive the first month of use And 95 percent survive five years of use Weibull is a distribution one of many Weibull Lognormal normal exponential and many others are names of statistical distributions They are formulas that describe the pattern formed by time to failure data eg repair times and many other groups or types of data Instead of Weibull analysis you could easily also say Were going to conduct a Normal analysis In reliability work I often first explore a set of life data by fitting a Weibull distribution to the data and plotting the probability density function PDF and cumulative density function CDF Its a first look and not the end of the analysis Each distribution has four functions that are useful for reliability engineering work 1 Reliability function 2 Cumulative density function 3 Probability density function 4 Hazard function Since I tend to like being positive about a product I often use the reliability function calculated at specific points in time t instead of the CDF which is the probability of failure over time t The reliability function is a function of time hence my suggestion to always include a probability and duration when specifying or reporting reliability values Weibull is a distribution not a metric The Weibull distribution like other distributions is a curve or equation It is not a metric on its own Define the time intervals of interest run out the calculations I recommend using the reliability function for the appropriately fitted distribution and then you have a metric Goals are not metrics yet they should be something we can measure that help us make better decisions For example setting reliability goals for one month the warranty period and during the expected use life Then use vendor or testing data and or field data to estimate the distribution of the life data Then again for specific time intervals of interest calculate reliability

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