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# Standard deviation watch

1. I don’t know what SD formula to use...
3. (Original post by Reece.W.J)
No need to bump threads a few moments after creation Are you talking about,

vs.

4. (Original post by Reece.W.J)
I don’t know what SD formula to use...
It is nearly always best to use this formula:

(Or an equivalent formula with frequency).

You may also be able to find it using your calculator. Are you doing new spec and do you have an example question?

5. (Original post by Notnek)
It is nearly always best to use this formula:

(Or an equivalent formula with frequency).

You may also be able to find it using your calculator. Are you doing new spec and do you have an example question?

New spec. I’ve done the whole table but is the sigma 4 (4 rows down) or 220 (total f)?
6. (Original post by Reece.W.J)
New spec. I’ve done the whole table but is the sigma 4 (4 rows down) or 220 (total f)?
I don't know what you're talking about (but maybe someone else will). You haven't posted an example question so I don't know where these numbers are coming from.
7. (Original post by Notnek)
I don't know what you're talking about (but maybe someone else will). You haven't posted an example question so I don't know where these numbers are coming from.
8. (Original post by Reece.W.J)
Might help using the formulae to label your sums at the bottom as , etc.
9. (Original post by _gcx)
Might help using the formulae to label your sums at the bottom as , etc.
Is midpoint Xi or X?
10. (Original post by Reece.W.J)
It looks like you are dealing with a grouped frequency table here (you should have made your question more clear initially) so you can only estimate the standard deviation.

So for a group like e.g. you assume that all the data points are equal to 15 i.e. the midpoint. The formula to use is

So something like is the sum of all the midpoints multiplied by the corresponding frequencies so that's the sum of your f x MP column.
11. (Original post by Reece.W.J)
I don’t know what SD formula to use...
Remember: there's 2 types... Sample and Population.

Are you doing WJEC spec by any chance?
12. (Original post by Notnek)
It looks like you are dealing with a grouped frequency table here (you should have made your question more clear initially) so you can only estimate the standard deviation.

So for a group like e.g. you assume that all the data points are equal to 15 i.e. the midpoint. The formula to use is

So something like is the sum of all the midpoints multiplied by the corresponding frequencies so that's the sum of your f x MP column.
Ok I don’t really get it when it’s typed out like that. Should I do root 220(3590-16.32)^2/5-1?
13. (Original post by RoyalSheepy)
Remember: there's 2 types... Sample and Population.

Are you doing WJEC spec by any chance?
Yes WJEC but we’ve done 3 types (small easy data, one with f, one w/o f)
14. (Original post by Reece.W.J)
Ok I don’t really get it when it’s typed out like that. Should I do root 220(3590-16.32)^2/5-1?
It will be worth you getting used to this notation. I'll rewrite it in words

15. (Original post by Reece.W.J)
Ok I don’t really get it when it’s typed out like that. Should I do root 220(3590-16.32)^2/5-1?
It's not clear where all of these numbers came from. Please post your working in full.
16. (Original post by Notnek)
It's not clear where all of these numbers came from. Please post your working in full.
17. (Original post by Reece.W.J)

No I don't, sorry! 2 secs...
18. (Original post by Notnek)
19. mean of the squares take away the square of the mean is a nice way to remember variance formula
20. (Original post by Reece.W.J)

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