Page 178 - Safety Risk Management for Medical Devices

CHAPTER 17

Risk Estimation

Abstract
According to ISO 14971 risk is a combination of the probability of occurrence of Harm and the
severity of that Harm. Three methods are presented for risk estimation: qualitative, semi-
quantitative, and quantitative.
Keywords: Risk estimation; qualitative; semi-quantitative; quantitative

According to ISO 14971 [3,7] risk is a combination of the probability of occurrence
of Harm and the severity of that Harm.
Three methods are commonly used to estimate the risk of Harm. In increasing
order of preference, they are: Qualitative, Semi-quantitative, and Quantitative. Each
method is described below.

17.1 QUALITATIVE METHOD
The qualitative method is used when quantifiable data is unavailable, or confidence in
the available data is low. In such cases use an N 3 M matrix such as the example in
Fig. 17.1 to stratify the risks. In this example a 3 3 3 matrix is used to stratify the risks
into three zones: high (red), medium (yellow) and low (green).
In order for this method to work, very good definitions for each probability and
severity level should be given to ensure repeatability and consistency of ratings by dif-
ferent analysts, at different times. Tables 17.1 and 17.2 offer examples of language that
could be used to promote consistency in severity and probability ratings.

17.2 SEMIQUANTITATIVE METHOD

The semiquantitative method is similar to the qualitative method, but with the differ-
ence that data is available for probability of occurrence of Harm. Generally, this is
true for products that have been in the field for a significant length of time and about
which a lot of field data has been collected.
The scales for probability of occurrence of Harm would be different for different
products. Examples: “probability of harm per use,” “probability of harm per device,”
“probability of harm per hour of use.”

Safety Risk Management for Medical Devices r 2018 Elsevier Ltd.
DOI: https://doi.org/10.1016/B978-0-12-813098-8.00017-9 All rights reserved. 157