Why rounding-off numbers is necessary

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

28 November 2021

Raxy Ngure from Alliance High School is uncomfortable with the practice of rounding off numbers. He writes: “Rounding off can take or add very large sums to a value. For example, (according to Google) the distance from Earth to Sun is 147.75 million kilometres. However, in the geography textbook says it is 150 million kilometres. I understand the logic of rounding off but is it a must to do so and get a value 2.25million kilometres more? Couldn`t they just write 147.75million kilometres? 2.25 million kilometres is large distance to be ignored in the name of rounding off. Or they could just round off to147.8 million kilometres.”

I have discussed this matter in the past. In a 2005 article right here, I wrote “It is common to hear, a government minister reading a statement that goes: “…we are now releasing two billion, three hundred and eighty six million, nine hundred and forty two thousand, seven hundred and twelve shillings to be used in the three thousand, one hundred and eighty five projects…If you’ve lost track of those numbers, read the paragraph again, this time with a pen and paper at hand!”

That paragraph illustrates one of the reasons why it is necessary to round off numbers: brevity. Reading out the exact numbers (Sh2,386,942,712 to be used in 3,184 projects) in full only adds confusion. It would be better to say Sh2.4B to be use in 3,000 projects.

The second (and probably more important) reason why numbers need to be rounded off is the fact that no measurement is accurate. This is the golden rule of experimental science.

Ngure’s writes that, according to Google, the distance to the sun is 147.75 million kilometres while the Geography textbook says 150 million km. Well, both sources are wrong! The distance is not a constant value. It varies from about 147.095 million km to about 152.1 million km depending on the Earth’s location on its journey around the sun. Remember: the orbit is not circular, it is elliptical. I have used the word “about” to emphasise the fact that even the 147.095 and 152.1 are approximations.

Perhaps it would be clearer to state “from about 147 million to about 152 million km”. This brings out the fact that the variation is about 5 million km more sharply. Clearly, then, we shouldn’t let the numbers obscure the magnitude of a quantity!