Precision, Efficiency, and Our Obsession with Round Numbers

Humans are remarkably good at estimating magnitudes. For example, we can estimate the amount of time it will take us to drive from point A to point B, so we know what time we should leave. We estimate how wide to cut each slice of cake so that everyone gets a piece. And we estimate how many sheets of paper towel it will take to clean up a spill.

To perform these feats of estimation, we rely as much on our natural estimation abilities as we do on our knowledge and experience. We learn from our mistakes, so when we realize we are consistently under- or over-estimating a quantity, we adjust and improve.

However, there is a competing factor that consistently and severely interferes with our ability to make accurate estimates: a compulsion to use round numbers.

Reheating Coffee

To give a personal example, I tend to drink a couple of cups of coffee each day. When I open the microwave to reheat a cup, I have a choice of how long to reheat it for. From experience, my 1100-watt microwave takes approximately 42 seconds to heat my coffee to the ideal heat level. (For my wife, it's closer to 53 seconds.) Yet I am much more likely to simply reheat the coffee for an even 30 seconds, or a full 1 minute (both of which can be done by pressing a single button), rather than go through the two or three extra button presses it takes to set an exact time like 42 seconds.

The end result is that I end up with either lukewarm coffee, or piping hot coffee that I need to let cool down for several minutes before I drink it.

In fact, my inclination to use the “easy” 30-second or 1-minute option is so strong, that I often adjust how much coffee I pour into my cup to match the heating time. I may end up pouring a smaller cup or a larger cup than I needed, just so I can heat it properly with a single touch.

The Ubiquity of Round Numbers

The coffee example may seem a bit facetious; after all, there are no major consequences of drinking coffee that is a little too cold or a little too hot. But our compulsion to use round numbers is ubiquitous and may affect other aspects of our lives.

When doctors prescribe a new medication to a patient, they may opt for a 5mg dose or a 10mg dose, but never a 8.2mg dose (even if 8.2mg would be the ideal dose for the patient, given their weight or the severity of their condition). Similarly, when we give our own dog anti-flea medication, we need to decide between a 10mg dose for small dogs, or a 20mg dose for medium dogs. But our dog is actually medium-small sized; shouldn't we use a dose of about 13mg? Unfortunately, pharmacists don't make arbitrary-size doses, so we end up rounding up or rounding down.

There are good reasons for society to provide consistent, discrete quantities for certain things. It would be inconvenient, for example, if stores carried 20 different size options for vitamin supplements, so customers could choose the precise size for their body weight. Providing a few limited, standardized size options not only makes easier for manufacturers, but also removes the burden on the consumer to choose from among too many options. When there are too many options, consumers may feel overwhelmed and experience choice overload (see Scheibehenne, Greifeneder & Todd, 2010 for a review).

Nevertheless, it is useful to recognize instances where it is simply up to us to make an accurate estimate. It doesn't take any longer to type 12 or 10, so go ahead and tell your friend you are running 12 minutes late. Go ahead and microwave your coffee for 42 seconds. Relinquishing your attachment to round numbers might just set you free.