I just read through a 20 or 30 comment thread/rant on a newsgroup about how Excel can’t add up because .29 – .28 doesn’t equal .01. (when you expand it out beyond Excel’s published accuracy limits (which comply with the IEEE standards)).
The original poster asserts repeatedly that even kids know .29 – .28 = .01000000.
That statement is so grossly wrong it alarms me. So much so that I was going to comment on the thread – except its a few days old and no one would notice probably.
Never mind the floating point approximation issue that flares up regularly (which was the cause of his initial rant), this is much more fundamental.
Its basic maths, accuracy and significant digits, and approximation.
.29 is the way of writing any number from .285 to less than .295
.28 is anything from .275 to less than .285
which means one minus the other can fairly be anything between (but not including) 0 and .02.
I don’t think you can justifiably have an answer to a greater precision than the inputs, can you?
I see lots of multi year projections down to the pence, it just seems total false security to me, better to be approximately right than precisely wrong (IMO). That’s why I like using =round() with a negative second argument to get round thousands, millions or billions.
Do you see much of this fantasy accuracy?
(and yes Excel can add, and take away – within its documented accuracy limitations – which should be adequate for most general purposes)