class reliability.Reliability_testing.reliability_test_planner(MTBF=None, number_of_failures=None, CI=None, test_duration=None, one_sided=True, time_terminated=True, print_results=True)

The function reliability_test_planner is used to solves for unknown test planning variables, given known variables. The Chi-squared distribution is used to find the lower confidence bound on MTBF for a given test duration, number of failures, and specified confidence interval.

The function will solve for any of the 4 variables, given the other 3. For example, you may want to know how many failures you are allowed to have in a given test duration to achieve a particular MTBF. The user must specify any 3 out of the 4 variables (not including one_sided, print_results, or time_terminated) and the remaining variable will be calculated.

  • MTBF (float, int, optional) – Mean Time Between Failures. This is the lower confidence bound on the MTBF. Units given in same units as the test_duration.
  • number_of_failures (int, optional) – The number of failures recorded (or allowed) to achieve the MTBF. Must be >= 0.
  • test_duration (float, int, optional) – The amount of time on test required (or performed) to achieve the MTBF. May also be distance, rounds fires, cycles, etc. Units given in same units as MTBF.
  • CI (float, optional) – The confidence interval at which the lower confidence bound on the MTBF is given. Must be between 0.5 and 1. For example, specify 0.95 for 95% confidence interval.
  • print_results (bool, optional) – If True the results will be printed. Default = True.
  • one_sided (bool, optional) – Use True for one-sided confidence interval and False for two-sided confidence interval. Default = True.
  • time_terminated (bool, optional) – Use True for time-terminated test and False for failure-terminated test. Default = True.

  • MTBF (float) – The lower bound on the MTBF.
  • number_of_failures (int) – The number of failures allowed to achieve the MTBF at the specified CI and test_duration
  • test_duration (float) – The required test duration
  • CI (float) – The confidence interval.


Please see the documentation for more detail on the equations used.

The returned values will match the input values with the exception of the input that was not provided.

The following example demonstrates how the MTBF is calculated:

from reliability.Reliability_testing import reliability_test_planner
reliability_test_planner(test_duration=19520, CI=0.8, number_of_failures=7)
>>> Reliability Test Planner results for time-terminated test
>>> Solving for MTBF
>>> Test duration: 19520
>>> MTBF (lower confidence bound): 1907.6398111904953
>>> Number of failures: 7
>>> Confidence interval (2 sided):0.8