Classic “piece of string” uncertainty example

Let’s take a simple example to illustrate the measurement uncertainty in practice; we give the same piece of string to three different people (one at a time) and ask them to measure the length of that string. There are no additional instructions given. They can use their tools and methods to measure it.

More than likely, you will get three somewhat different answers. For example:

• The first person says the string is about 60 cm long. He used a 10 cm plastic ruler and measured the string once and came to this conclusion.

• The second person says it is 70 cm long. He used a three-meter measuring tape and checked the results a couple of times to make sure he was right.

• The third person says it is 67.5 cm long with an uncertainty of ±0.5 cm. He used an accurate measuring tape and measured the string several times to get an average and standard deviation. Then, he tested how much the string stretched when it was pulled and noticed that this had a small effect on the result.

Even this simple example shows that many things affect the result of measurement: the measurement tools that were used, the method/process that was used, and the way that the person did the job.So, the question you should be asking yourself is:

At your plant, when calibration work is performed, which of these three above examples will it be?

What kind of “rulers” are being used at your site and what are the measuring methods/processes?

If you just measure something without knowing the related uncertainty, the result is not worth much.