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# Standard Deviation Watch

1. (Source: REVISE EDEXCEL GCSE Statistics Revision Guide)

Hello,

Please, can you help me solve this question about the mean, variance and standard deviation:

You have to use: √Σx²/n - (Σx/n)²

The question is, the number of visits (x) to a dentist was recorded over 10 days.
Σx = 200
Σx² = 480
Work out the mean, the variance and the standard deviation.

Thanks
2. (Original post by sunflowerlion)
(Source: REVISE EDEXCEL GCSE Statistics Revision Guide)

Hello,

Please, can you help me solve this question about the mean, variance and standard deviation:

You have to use: √Σx²/n - (Σx/n)²

The question is, the number of visits (x) to a dentist was recorded over 10 days.
Σx = 200
Σx² = 480
Work out the mean, the variance and the standard deviation.

Thanks
It's recorded over 10 days so n is 10. You've been given Σx and Σx² and just sub these into the equations for mean, variance and standard deviation.
3. When I try to do that, it comes up with an error. I don't think I'm typing it in right.
4. (Original post by sunflowerlion)
When I try to do that, it comes up with an error. I don't think I'm typing it in right.
Are those numbers definitely correct? I tried subbing it in and i I got math error as well so I can only think the values are wrong.
5. (Original post by miless090)
Are those numbers definitely correct? I tried subbing it in and i I got math error as well so I can only think the values are wrong.
Yeah they are. I've just checked answers in back:
Mean = 20
Variance = 80
Standard deviation = 8.9
But I have no idea how they got that, there's no workings.

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6. The numbers can't be right- you can't square root a negative! The book might have made a typo- if you use Σx² = 4800 then you get the right standard deviation and variance
7. (Original post by somegirlcalledea)
The numbers can't be right- you can't square root a negative! The book might have made a typo- if you use Σx² = 4800 then you get the right standard deviation and variance
Thanks a lot There was a typo in the book.

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