When You Assume...

Posted by bob on March 4, 2024

There is a common cliché often used to warn students about making unfounded assumptions. Oddly, when we start solving any problem, we must make assumptions. The difference is that we expect those assumptions to be well-founded.

The five-question method of solving problems asks:

  • What do you know?
  • What are the rules?
  • What don’t you know?
  • How are you going to find out the things you don’t know?
  • How will you know when you are done solving the problem?

Clearly, when we state the things we know, we are stating assumptions. Some assumptions are self-demonstrating or are given to us as initial conditions. As the old saying goes, “it ain’t the things we don’t know that cause us trouble, it’s the things we know for sure that just ain’t so.” 1

When you make that list of things-we-know, it might be a good idea to state how-we-know-this or better yet, if you have verified that item.

This can lead down a rabbit-hole. For example, if you connect a new piece of equipment to AC line voltage, how often do you actually measure that incoming voltage? Sure, you know that USA line voltage is supposed to be around 120VAC, but often runs anywhere from 110VAC to 130VAC. You also need to be able to validate the calibration of your measurement instrument.

Many years ago, we connected a new video amplifier in a TV station. There was no output, but we could clearly see the input signal. We checked the internal DC voltages and found no useful voltage on any of the rails. Suspecting a power supply failure, we checked the input voltage and found that it was running about 135VAC. The power supply regulators had shut down based on an input over-voltage protection circuit. We added a little resistance to the AC input and found that the amplifier started working as expected. Tracing the AC wiring showed that this device was plugged-in closest to the AC source transformer and that virtually everything else in the facility was seeing lower AC line voltage.

When solving problems in school, many of the known items are givens. In other words, the professor tells you the voltage and current and asks you to solve for the resistance. When you are solving real-world problems, some of the known items might be givens from your customer. It can be important to consider if you can accept these as truly given facts or if you need to verify them in some way.

Some customers might react poorly to you questioning what they believe are facts, so you might need to tread lightly. Sometimes it helps to lead the customer to believe they have uncovered the issue and how grateful you are to build a better product in response.


1 This quote is frequently attributed to author Mark Twain. It appears to be a great saying that simply sounds more important when attributed to a famous and favorite author.