E
Engineering Guy
Hi All, long time reader, first time poster. I am a mech. engineer, frequently involved in "bench testing"of medical devices.
I have trauled through the forums many times over the last few years on sample size selection and never really get much wiser. I've asked as many colleagues as I can in my 7 years in the medical industry, but have never been given any good reference for developing a 'valid statistical rationale' for design verification. I have seen many good references for process capability and batch acceptance but never for design verification. To date I have been either using a standard which dictates sample size (e.g. sterilization validation and BioC) or somewhat arbitrarily picking a number that I think will represent the nominal design. Some of my testing has been submitted in 510(k)s or in EU submissions for class II devices. My sample size rationale has never been questioned, but more and more I hear people talking about valid statistical rationale.
So here is my current case:
I have a disposable medical inhaler device - for sake of arguement lets say its a gas mask which filters incoming air with activated carbon that you only use for 20 minutes before discarding - which i need to test for airway particulate. The device is already manufactured and sold. Batch sizes are between 25k to 100k. ISO 18562-2 defines a limit for airway particulate emitted by the device. That limit is 12 um/m^3 (12 micrograms of particulate, per cubic meter of air inhaled through the device). The standard says the recommended test methods are "Type Tests" which to me means 1 sample. However, given this is a disposable device, and the particulate is likely to vary - it actually does contain activated carbon granules which are assembled into the device manually - i am concerned that 1 sample would not represent the nominal design. What rationale can i use in selecting a sample size for this application?
I have done some preliminary tests which indicate the average is 4 um/m3 across 10 devices. Without getting into details - that test took 10 hours, and does not allow me to measure the particle density for individual devices, only the average across all.
Note that the above standard has only recently become recognized, hence why i am testing after the product is released (nothing dodgey here!)
TIA!
I have trauled through the forums many times over the last few years on sample size selection and never really get much wiser. I've asked as many colleagues as I can in my 7 years in the medical industry, but have never been given any good reference for developing a 'valid statistical rationale' for design verification. I have seen many good references for process capability and batch acceptance but never for design verification. To date I have been either using a standard which dictates sample size (e.g. sterilization validation and BioC) or somewhat arbitrarily picking a number that I think will represent the nominal design. Some of my testing has been submitted in 510(k)s or in EU submissions for class II devices. My sample size rationale has never been questioned, but more and more I hear people talking about valid statistical rationale.
So here is my current case:
I have a disposable medical inhaler device - for sake of arguement lets say its a gas mask which filters incoming air with activated carbon that you only use for 20 minutes before discarding - which i need to test for airway particulate. The device is already manufactured and sold. Batch sizes are between 25k to 100k. ISO 18562-2 defines a limit for airway particulate emitted by the device. That limit is 12 um/m^3 (12 micrograms of particulate, per cubic meter of air inhaled through the device). The standard says the recommended test methods are "Type Tests" which to me means 1 sample. However, given this is a disposable device, and the particulate is likely to vary - it actually does contain activated carbon granules which are assembled into the device manually - i am concerned that 1 sample would not represent the nominal design. What rationale can i use in selecting a sample size for this application?
I have done some preliminary tests which indicate the average is 4 um/m3 across 10 devices. Without getting into details - that test took 10 hours, and does not allow me to measure the particle density for individual devices, only the average across all.
Note that the above standard has only recently become recognized, hence why i am testing after the product is released (nothing dodgey here!)
TIA!