Reliability Model Worksheet Features 
The in-tolerance probability of parameters is ordinarily not constant with time elapsed since measurement. Because the uncertainty in the value of a parameter begins to grow with time, the in-tolerance probability tends to decrease with time.  In AccuracyRatio, the change in in-tolerance probability with time can be modeled using the Reliability Model Worksheet, which is accessed from the Main Screen or from the Parameter Bias Uncertainty Worksheet. 

In using the Reliability Model worksheet, we select a mathematical reliability model from a list of eight candidates that have been found useful in modeling the uncertainty growth behavior of measurement and test equipment.  In addition, there are two data entry options for defining the reliability model using the Reliability Model Worksheet. These options are described below.
 
Enter Reliability Model Coefficients
One approach to using the worksheet is to enter the results of interval analysis conducted on the parameter’s population. When entering interval analysis results, there is a tacit assumption that the subject parameter that was calibrated came from a population of like parameters, each of which was manufactured to the same specifications. ISG's IntervalMAX program, for example, fits various reliability models (e.g., gamma, Weibull, etc.) to the calibration history of like parameters and solves for reliability model coefficients.
 
Enter In-Tolerance Probabilities
If you don't have access to a calibration interval analysis program, a second method is to enter in-tolerance probabilities at the beginning and end of the calibration interval. For some reliability models, an average over period in-tolerance probability is also needed.
 
Reliability Model Plot
Once the reliability model has been defined, a plot of the in-tolerance probability versus time is displayed in the lower right portion of the Worksheet.  Default values are displayed in the Interval, Reliability and Plot Limit boxes located below the plot. The display value in the Interval box is the same as in the Cal / Test Interval box. The value in the Reliability box is the same as in the End of Period in-tolerance probability box. The value in the Plot Limit box is the display range for the horizontal plot axis and is automatically estimated by AccuracyRatio.

You can enter different values in the Interval, Reliability, and Plot Limit boxes without impacting the reliability model results. For instance, the Plot Limit value can be increased or decreased to enhance the plot detail as desired. The Interval value can be increased or decreased to assess the impact that a shorter or longer calibration interval will have on the end-of-period in-tolerance probability, which is recomputed and displayed in the Reliability box. Conversely, the Reliability value can be increased or decreased to assess what impact increasing or decreasing the end-of-period in-tolerance probability will have on the computed calibration interval.