July 13, 2021

ISO 14971 Estimating Probability of Occurrence of Harm

Based on the aforesaid flexibility (see my earlier blog post) allowed by ISO 14971 (e.g., see the third paragraph of ISO/TR 24971 section 5.5.2) for how to estimate probability of occurrence of harm, one might employ a variety of probability expressions.  For example, occurrence of harm per day, occurrence of harm per year, occurrence of harm per unit, occurrence of harm per use, occurrence of harm per patient, occurrence of harm per age group, occurrence of harm during the device lifetime, etc.

To help illustrate, here are some hypotheticals:

• If a particular harm is estimated to happen once per day with a vital signs monitor model, then the probability would seem to be different (higher) than if that harm is estimated to happen only once per year.

• If a particular harm is estimated to happen once per device (e.g., per use, per unit sold, etc.) for a single-use device, then the probability would seem to be different (higher) than if that harm is estimated to happen only 0.001 times per device (e.g., per use, per unit sold, etc.).  For example, one occurrence in 100 units sold = 0.01 occurrences/unit sold, whereas one occurrence in 100,000 units sold is only 0.00001 occurrences/unit sold.

• Or as another example, 100 occurrences over a ten-year lifetime = 10 occurrences / year, whereas 1 occurrence over a ten-year lifetime is only 0.1 occurrences per year.

• Or as a qualitative example:  Likely to happen several times during a ten-year device lifetime would be a higher probability than not likely to happen during a ten-year device lifetime (see Table 3 of ISO/TR 24971).

I have seen some who disagree with this logic by asserting that the probability rate doesn’t change based on sales or the size of the installed base, and that the number of devices doesn’t change the rate, and that the rate doesn’t change based on the lifetime of the device).  However, I believe such a stance would be at odds with the principles of ISO 14971.