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When should you round for S1/S2?

When using the formula z=(x - mean)/sd and you get a figure more than 2 decimal places when should you round?

If I had 1.525 would I use 1.53 off the table or 1.52 what about 1.5266 etc. (exam board is edexcel if that matters).

Thank you
They will accept a range of answers, so in that case you could use either 1.52 or 1.53. Or you could use the stats mode on your calculator and find probabilities using the unrounded standardized Z value.
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Avatar for Ayman!
Ayman!
15
Original post by SugarCoatedCart
When using the formula z=(x - mean)/sd and you get a figure more than 2 decimal places when should you round?

If I had 1.525 would I use 1.53 off the table or 1.52 what about 1.5266 etc. (exam board is edexcel if that matters).

Thank you


How would you round 1.525 to 2 DP normally? If you intend on using tables, then you round to 2 DP and then use the table values. If you round incorrectly they'll dock accuracy marks even if your answer falls within the acceptable range.
Original post by NotNotBatman
They will accept a range of answers, so in that case you could use either 1.52 or 1.53. Or you could use the stats mode on your calculator and find probabilities using the unrounded standardized Z value.


Didn't know a calculator could do that - thanks!

Original post by Ayman!
How would you round 1.525 to 2 DP normally? If you intend on using tables, then you round to 2 DP and then use the table values. If you round incorrectly they'll dock accuracy marks even if your answer falls within the acceptable range.


1.53, thanks for the help :smile:
Reply 4
Avatar for MathsSir
MathsSir
1
The only issues surrounding rounding are to do with the use of a number in subsequent calculations.
Always use as many as possible when calculating regression coefficients as the questions are chosen such that rounding any of the values Sxx, Sxy and Syy as well as xbar and ybar can give a wrong answer (in 2nd 3rd or 4th decimal place) if rounded too early. This will have a knock-on effect for later parts to a question. Also, unless stated otherwise, give PMCC to 3dp.

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