# 2+2 ≠ 4

What do NASA’s Mars Climate Orbiter spacecraft, modular arithmetic, and some medical errors have in common?

It's worth clarifying what we measure with. Not just what we measure.

We might say the same thing and mean completely different things.

Sentiments like “we’ll be fine in six months”, “that was a huge success” mean nothing if we can’t clearly define what you mean by ‘being fine’ and what metric defines success.

The units matter. So does context.

Like someone jokingly told me a year ago, 2 drops of water added to 2 drops of water = 1 drop of water. 2 hungry cats plus 2 sausages = 2 happy cats.

There are cases where 2+2=100.3 Where 2+2=0 and 2+2≡1.4

For 2+2=4 to be true, you only need to confirm the unit you have in mind - whole numbers in standard arithmetic.

The units always matter.

We might say the same thing, but mean completely different things.

It’s up to you not to make an assumption. Not for you. Not for anyone.

NASA lost the $125-million Mars Climate Orbiter spacecraft in 1999 because of a simple communication error. Two teams didn’t speak the same metric language in their calculations and designs. The navigation team built their calculations using millimeters and meters, and the team that designed and built it used inches, feet, and pounds. We now know what that led to.

Case in point: a two-year-old boy received an overdose because his weight was entered as 35 kilograms instead of 35 pounds. For context, 35 kg is 77 pounds.

Like in the binary numbers system where 2 is represented as 10. 10+10=100.

Numbers in modular arithmetic ‘wrap around’ after reaching a certain value. In mod 2, 2+2=0, since there’s no remainder, and 2+2 ≡ 1 in mod 3 arithmetic.