Abstract

Chris and Fred discuss what it means to ‘assume’ something. We assume things all the time in our daily lives. We would never get anything done if we didn’t rely on quick ‘guesses’ about the world around us. But then there are those ‘assumptions’ that get us in trouble. So what’s the difference?

Key Points

Join Chris and Fred as they discuss what it means to ‘assume’ something. Engineers often love making assumptions. Why?

Topics include:

• What is an ‘assumption?’ An assumption is essentially free information. It is ‘free’ because you don’t have to go and acquire it (as a rule). So you can (for example) assume that corrosion is the dominant failure mechanism of a component. That might then mean that all you need to do to ensure that it is reliable is to ensure that we protect the component from moisture (and other things that cause corrosion to occur). This is a good assumption if you can explain a valid rationale based on genuine experience. There is no need to test things for conclusions that you can come up with yourself.
• What is a ‘bad’ assumption? Assuming that products fail with a constant hazard rate (for example). This implies that your product doesn’t wear out, or wear in. In other words, a product that is working 100 years from now is just as likely to fail by the end of the day as a brand-new product.
• Why are we tempted to make bad assumptions? It always comes down to people wanting to create the perception of progress, sometimes at the expense of actual progress. Assuming a constant hazard rate for products makes all the mathematics much easier. Which means you can do things like fill in spreadsheets, write reports, predict reliability and lots of other things really quickly. The problem is that these numbers are largely meaningless because pretty much nothing has a constant hazard rate.
• You can assume a bull is a cow, but now matter how much you milk it, you won’t get any milk.

Enjoy an episode of Speaking of Reliability. Where you can join friends as they discuss reliability topics. Join us as we discuss topics ranging from design for reliability techniques to field data analysis approaches.

SOR 926 Just AssumingChristopher Jackson

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