# OCR MEI S1 - SO CONFUSED. How many sig. figs do you round to in your answers?Watch

Thread starter 2 years ago
#1
I always get the correct answers for working out, say a mean or a standard deviation, BUT I NEVER ROUND IT to the correct number of figures. Is there any rules for this? Does anyone have any tips/advice?
0
2 years ago
#2
Sorry you've not had any responses about this. Are you sure you've posted in the right place? Here's a link to our subject forum which should help get you more responses if you post there.

You can also find the Exam Thread list for A-levels here and GCSE here.

Just quoting in Puddles the Monkey so she can move the thread if needed
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(Original post by Puddles the Monkey)
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0
2 years ago
#3
(Original post by melissadrake)
I always get the correct answers for working out, say a mean or a standard deviation, BUT I NEVER ROUND IT to the correct number of figures. Is there any rules for this? Does anyone have any tips/advice?
Hi! I moved this into the maths forum for you - you're more likely to get a response here
1
2 years ago
#4
To quote from the 2013 OCR MEI S1 Mark Scheme (available here):

(Original post by OCR)
Candidates are expected to give numerical answers to an appropriate degree of accuracy. 3 significant figures may often be the norm for this, but this always needs to be considered in the context of the problem in hand. For example, in quoting probabilities from Normal tables,we generally expect some evidence of interpolation and so quotation to 4 decimal places will often be appropriate. But even this does not always apply – quotations of the standard critical points for significance tests such as 1.96, 1.645, 2.576 (maybe even 2.58 – but not 2.57) will commonly suffice, especially if the calculated value of a test statistic is nowhere near any of these values. Sensible discretion must be exercised in such cases.

Discretion must also be exercised in the case of small variations in the degree of accuracy to which an answer is given. For example, if 3 significant figures are expected (either because of an explicit instruction or because the general context of a problem demands it) but only 2 are given, loss of an accuracy ("A") mark is likely to be appropriate; but if 4 significant figures are given, this should not normally be penalised. Likewise, answers which are slightly deviant from what is expected in a very minor manner (for example a Normal probability given, after an attempt at interpolation, as 0.6418 whereas 0.6417 was expected) should not be penalised. However, answers which are grossly over- or under-specified should normally result in the loss of a mark. This includes cases such as, for example, insistence that the value of a test statistic is (say) 2.128888446667 merely because that is the value that happened to come off the candidate's calculator.

Note that this applies to answers that are given as final stages of calculations; intermediate working should usually be carried out, and quoted, to a greater degree of accuracy to avoid the danger of premature approximation.

[...]

NOTE RE OVER-SPECIFICATION OF ANSWERS
If answers are grossly over-specified, deduct the final answer mark in every case. Probabilities should also be rounded to a sensible degree of accuracy. Ingeneral final non-probability answers should not be given to more than 4 significant figures. Allow probabilities given to 5 sig fig
So, essentially, 3 or 4 sig fig will normally be fine. However, context is the key - just try to give your answers to a similar level of accuracy as given in the questions.
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