tolerances and failure trends

Figure 13.2 illustrates such a change in performance with time.

Figure 13.2 Change in performance represented by a linear change of a parameter over a time period t after which corrective action is taken to restore the performance.

Considering the effect of the length of the time period t:

If t was one day then, depending upon when a patient was treated and assuming they attended at the same time each day, then their dose for the entire treatment will be different by between 0% and −3% over the full course of treatment.

For all treatments of duration greater than or equal to 5 days, the variation from the required dose will be approximately −1.5%. For treatments of 4 days or less, the variation will be between 0% and −3% depending upon the day and the number of days treated. For example, a 3-day treatment on days 2, 3 and 4 will have a mean difference of −(0.6% + 1.2% +1.8%)/3 = −1.2%. However, an 8-day treatment on days 1 to 8 will have a mean difference of −1.575%

For treatments of duration greater than or equal to 20 days, the variation from the required dose will be about −1.5%. For treatments of 19 days or less, the variation will be between 0% and −3% depending upon the day and the number of days treated.

Hence the length of t can have a variable effect on the dose delivered to the patient.

If t is long, say several times as long as a total course treatment time, T, where typically that can be between 3 to 6 weeks, the following occurs compared to having t equal to T. The variation in the dose is shown in Figure 13.3.