Rounding in IB exams

Hey guys,

I was just going through some previous IB exams and I noticed that when they did any steps to get to the answer, they rounded the answers from those steps to the correct sig figs, and they then round the final answer to the correct number of sig figs again. I was always taught to carry through all answers and then round the final answer to the sig figs. So I was wondering, will I be penalized if I only round the final answer and I'm off by a bit?
Also, when I use a different number for the constants, ie gravity (they sometimes use 10N/kg and I use 9.81N/kg) will I lose marks?

It may seem that I'm paranoid but any answers will be appreciated.

Thanks.

Reply 1

IB-more

All my math tests say on them three significant digits. It always confused me too, that if you're rounding between steps, you're not really getting the real answer. They do look at your work, however, so if you used 9.81 ms-2, and they use 10 ms-2, I think it should be ok.

We use 9.80 for some reason. I don't know why.

Reply 2

steve2005

Guyofpie

I think you should round at the end. As far as constants are concerned , look at the instructions... they often tell you what is required.

Reply 3

nota bene

You are ONLY to round the answer (well you may round in your working, but not on the calculator so you get wrong answer).

All constants are given in the data booklets for physics/chemistry etc. Use those! However somehow, for simplicity, they instruct you to use e.g. g=10 in physics papers, but that will say so in the question, then do so :smile:

(although I doubt you'd lose marks for using the 9.81)

Reply 4

HMSChocolate

You round the final answer only!! Use the memory function on your calculator to carry over decimals!

As for g, well sometimes they tell you outright to use 9.81 or 10 but if they don't I usually just use whatever it says in the booklet (can't remember which one, 9.81 or 10)

Reply 5

-Aimee-

NAUTE - Never Approximate Until The End :tongue:

if you read the markscheme for physics carefully you will see that they will accept answers within 1 sig fig of the real answer. ie real answer = 2.53 they will accept 2.5 and 2.531 so i think you can have some marginal inaccuracies

Reply 6

Guyofpie

Oh ok, thanks for all your fast and helpful replies!

Reply 7

HMSChocolate

aimee_22

NAUTE - Never Approximate Until The End :tongue:

Unless they specifically say correct to 3sf or whatever

Reply 8

rawkingpunkster

I believe that if you round of intermediate answers and carry through the inaccuracy, IB SHOULD give the mark.

To be safe, you could indicate a rounded off intermediate answer, but use the exact ans when pressing in your calculator.

Reply 9

Kongo

Yeah, ive been having tons of problems with this on the physics past papers, firslty they normally use 10 for gravity.

Seconly if you for example find the answer to question A and then round it, youll get the right answer, but often you are required to re use that answer in part B and i have a tendancy to use the non rounded answer (still stored on my calculator) which gives me the wrong answer (eg by 0.1). Should i use the rounded answer from part A always?

And what is the accepted number of sig fig? Its easy in math where its always 3, but in physics i find they approximate almost the whole time. Grrr.

Reply 10

-Aimee-

i know what you mean :s-smilie:

like they want you to show that a value equals x, then you calculate it and its just about x (with 5 million more decimal places). then in the next question they ask you to do a calculation involving x, so you use the value you calculated instead of the one they ask you to show.
and then its WRONG!!
GRRRRRR!!

Reply 11

nota bene

I doubt they'd deduct points for that... (i.e. if you answer A with rounding and then carry on B without rounding until the final answer)