A location parameter can be used with a lognormal distribution or any case of the Weibull distribution. The location parameter, also called t-zero (t 0 ), defines the location of the distribution in time. The location parameter provides for shifting the origin of the age scale.

When using a location parameter, a value is either subtracted from or added to each age value. For example, if the probability of failure is zero for some given period of time, prior to generating a Weibull graph, the origin of the age scale should be shifted from zero to this time to reflect the guaranteed failure-free period. The correction, would be a positive value equal to the minimum time necessary for a failure to occur. However, a location parameter is only used if an earlier analysis using a two-parameter Weibull analysis has shown that a time shift is needed.

If a location parameter (t 0 shift) exists, then f (t) = 0 and R (t) = 1 for all values of t < t 0>. The modified probability density function, (t), and reliability, R (t), for the values t <t 0 are f (t - t 0) and R (t - t 0).

To provide for some loss of life (reliability) before service officially begins, a negative value can be entered. Negative corrections are helpful for situations where spare parts deteriorate while in storage. Rubber parts, chemicals, and ball bearings, for example, all deteriorate with prolonged storage. When the value applied to the data is correct, the resulting plot follows a straight line. Without prior experience, at least 20 failures are usually needed to perform a distribution analysis using the three-parameter Weibull distribution.

For more information about using a location parameter, see Threshold Parameter.