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Rounding up/down

The exam boards seem quite fussy about rounding to the right number of decimal places or significant figures. However, I'm not sure whether you round your numbers at every opportunity when doing a calculation, or whether you simply do it at the end of your calculation.

Could someone please help me with this?
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Avatar for Arbolus
Arbolus
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Original post by Default Human
The exam boards seem quite fussy about rounding to the right number of decimal places or significant figures. However, I'm not sure whether you round your numbers at every opportunity when doing a calculation, or whether you simply do it at the end of your calculation.

Could someone please help me with this?


Don't round until the end. If you round at every stage of the calculation, it will introduce inaccuracies which might cause the final answer to end up being significantly different.
Original post by Default Human
(...)
Could someone please help me with this?


What should be calculated, if I may ask?

If your calculation process contains many steps and it must be very accurate, I would round at the last one to be close as possible.
Original post by Kallisto
What should be calculated, if I may ask?

If your calculation process contains many steps and it must be very accurate, I would round at the last one to be close as possible.


Thanks for the help!

I was just meaning generally. But I was unsure after doing the Unit 5 June 2012 paper, question 6d ii)

The mark scheme seemed to suggest that the mark would only be awarded if the number was rounded to 3 sf, despite the fact it was at the start of the calculation.

But, thinking about it, I think the mark scheme was saying that anything less than 2 sf wouldn't get the mark, rather than saying anything more than 3 sf would lose the mark.
Original post by Default Human
Thanks for the help!

I was just meaning generally. But I was unsure after doing the Unit 5 June 2012 paper, question 6d ii)

The mark scheme seemed to suggest that the mark would only be awarded if the number was rounded to 3 sf, despite the fact it was at the start of the calculation.

But, thinking about it, I think the mark scheme was saying that anything less than 2 sf wouldn't get the mark, rather than saying anything more than 3 sf would lose the mark.


I have rounded results - no matter whether in chemistry or other subjects - to two decimal places and I have had no problems at all.

As a rule I have calculated quantities of concentration in stochiometry, and there it would be enough to round to two decimal places at the last step of your calculation.
Reply 5
Avatar for Arbolus
Arbolus
13
Original post by Default Human
Thanks for the help!

I was just meaning generally. But I was unsure after doing the Unit 5 June 2012 paper, question 6d ii)

The mark scheme seemed to suggest that the mark would only be awarded if the number was rounded to 3 sf, despite the fact it was at the start of the calculation.

But, thinking about it, I think the mark scheme was saying that anything less than 2 sf wouldn't get the mark, rather than saying anything more than 3 sf would lose the mark.


Can't be specific without seeing the question. But what I would do, if you're asked to give a number from the start of the calculation, is to write down the rounded version as your answer but keep the unrounded version to one side and use that for the next stage of the calculation.

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