Page 201 - Safety Risk Management for Medical Devices

180 Safety Risk Management for Medical Devices

• Verification testing
In the domain of Risk Management, verification testing is performed to dem-
onstrate that Risk Controls are implemented as intended.
• Validation testing
In the domain of Risk Management, validation testing is performed to demon-
strate that the Risk Controls are effective in reducing risk.
• QC testing
This type of testing is relevant to the manufacturing process. It can be used as a
Risk Control to prevent defective parts/products from escaping to the field.

21.2 RISK-BASED SAMPLE SIZE SELECTION
It is wise to choose the rigor of your testing based on the safety risks related to the
subject of the test. Choice of the sample size is an indication of the level of rigor. A
higher sample size connotes higher confidence in the test results. This strategy can
also have an economic benefit in that for low-risk test subjects, we can reduce the
sample size and thereby save on product development costs.
The main question is: how many samples should be used for testing a safety-
related requirement. Before we dive in to the methodology, it is beneficial to define
two important terms: Confidence, and Reliability.

Confidence (1 2 α)—Probability that if the test passes, the requirement is met by at
least R% of the population, where R is the Reliability.

Safety-requirements can be tested as variable or attribute. Attribute testing results
are discrete, e.g., true/false or Pass/Fail, whereas variable test results are numerical on
a scale. In the following sections, a strategy is offered on how to make risk-based deci-
sions on sample sizes.

21.3 ATTRIBUTE TESTING

Requirements that are attribute-tested produce results that are discrete, usually binary. For
example, the requirement: “The implantable device shall be able to withstand a 3T MRI
environment without damage” would be attribute-tested. Multiple samples would be
exposedtoa3T MRIenvironments, andthentestedtosee if they suffered anydamage.
Using the BXM method, the following steps can be utilized to make a risk-based
determination of sample sizes for attribute testing. The core idea is to identify the risk
levels associated with the safety requirements and then determine the sample sizes
commensurate with their risk levels. Safety requirements are those requirements that
realize the Risk Controls.