How does it work?
When speaking about probabilities in the qualitative form, there is one commonality amongst all industries regardless of the risk level or total number of units sold per year. The common attribute is the “IMPROBABLE”, representing one (1) failure, complaint or nonconformity. The improbable serves as the base, the foundation in which the QQ Chart is built on. For products with low complexity in design or low risk profile that are manufactured and sold in millions (1,000,000) of unit per year. The "IMPROBABLE" risk criteria is 1 x 10 to the minus 6.
Using the same fundamental principle of “IMPROBABLE” as the base, representing one failure or complaint. For products with moderate complexity in design or moderate risk profile that are manufactured and sold in hundreds of thousands (100,000) of units per year. The "IMPROBABLE" risk criteria is 1 X 10 to the minus 5.
Similarly, following the same fundamental principle of “IMPROBABLE” which represents one failure or complaint. For devices with medium complexity in design or medium risk profile that are manufactured and sold in tens of thousands (10,000) of units per year. The "IMPROBABLE" risk criteria is 1 X 10 to the minus 4.
Last but not least, for complex or high risk profile devices manufactured and sold in thousands (1000) of units per year. The "IMPROBABLE" risk criteria is 1 X 10 to the minus 3.
Once the base has been established the other probability ranges are simply constructed one order of magnitude larger than the base. It’s important to realize that making risk-based decision solely based on the probabilities depicted in percentage could be erroneous. For instance, 1 nonconformity per 1,000,000 presented in percentage is 0.0001%. Similarly, 1 nonconformity in 10,000 would be 0.01%. In two separate industries, or two different types of products (single use tongue depressor and infusion pumps) both values represent “Improbable” yet depicted in percentages could make someone think otherwise. Therefore, it’s essential to focus on the number of failures, complaints or nonconformities instead of the percentages value alone.
About
Probability (P) can be presented both Qualitatively and Quantitatively, the QQ chart enables the user to easily without any calculation, convert the quantitative probability values gathered from the real-life data to the corresponding qualitative probability value set forth in the risk management document. The QQ Chart can be used but not limited to PHA (Preliminary Hazard Analysis) and FMEA (Failure Mode and Effect Analysis).
The QQ chart is an easy and simple tool that can be used by anyone across different disciplines and industries.
How to use the QQ Chart?
First, decide which risk criteria of "IMPROBABLE" is suitable for the device under analysis. Second, depending on which quadrant you have selected, simply move the lever on the circumference (outer edge) of the QQ chart looking for the total number of units under assessment. Once the lever points to the correct number of units, raise or lower the knob (red arrow) to align with the total number of failures or non-conformities. Look at the P value shown on the lever. This is the qualitative probability value (P) determined without any calculation.
Examples
(Download the App for an interactive experience)
What is the P value, If 27,300 units (medium risk profile) have been sold and there are 21 complaints?
Answer: P = 2
What is the P Value, If 273,000 units (moderate risk profile) have been sold and there are 120 complaints?