Standard deviation

Sorry I read your table wrong - I actually get 12.58. Your answer is not right. Can you please post your working in full?

I’ve got 196 now by doing root 8486500-220 x (3590/220)/219

I don't understand your working. What formula are you using?

The one in the formula booklet (probably the wrong one) and I might be putting the wrong numbers for the wrong thing such as X

If possible, can you please upload a screenshot of the formula you are looking at?

You seem to be getting mixed up with your formulas. In your working you have a table which includes e.g. f x MP. Where did this table come from? Are you following an example? Did your teacher give you a formula?

If you have time I recommend this video. It will explain the two main formulas.

Ik they should be called Xi and that but I just put midpoint on that. I’m aware of that formula and the other 2 but not sure how to fill it

I think you should start again and use this since your table indicates that you were trying to use this formula:

$\displaystyle \sigma^2 = \frac{\text{sum of frequency x midpoints}^2}{\text{sum of frequencies}}- \left(\frac{\text{sum of frequency x midpoints}}{\text{sum of frequencies}}\right)^2$

Firstly your $fMP^2$ column is wrong. $fMP^2$ means $f\times MP^2$ so you need to square the midpoint then multiply the result by the frequency.

So can you please fix your table then post a screenshot?

It seems like you will need to ask your teacher to clarify this topic to you.

The whole class performed poorly on this topic so she’s going over it but wants us to fix the 3 questions in the test before the lesson. I’ll get back to you I think I’m getting closer

Okay great. I'm sorry if my earlier posts confused you - I wasn't sure what you knew/didn't know. Plus helping with stats questions on TSR is quite new for me

Would you mind showing your working so I can learn to be able to do the next few? I’ve been doing this for like 1h now and I’m even getting maths error now

Sorry I don't have time to write out a solution but I can guide you through this one.

Your $fMP^2$ column is wrong. So for the first entry of that column you need to do $80\times 2.5^2 = 500$ so 500 is the frst entry instead of 40000. Can you fix the rest of them then work out the total and post your working?

We were told to do f x mp then square answer. As you can see it makes a big difference

Can you please explain in words how you were taught to do all this? It doesn't matter if what you say is wrong.

I don't want to give you a method that is different to your teachers. If you want to use a method that I suggest then let me know.

Thanks for all your help but I think I’m going to have to give up

Thanks for all your help but I think I’m going to have to give up

$\displaystyle \sigma^2 = \frac{\sum fx^2} {\sum f}- \left(\frac{\sum fx}{\sum f}\right)^2$

$=\displaystyle \frac{93425}{220} - \left( \frac{3590}{220}\right)^2 = 158.376...$

So $\sigma = \sqrt{158.376...} = 12.58$

Ok so I have that apart from the column on the right because we’ve definitely been told to square the answer of fx not f x x^2

I've shown you a method that will give you the answer. If your teacher is using a different method then please check with them.