A-Level Maths: L3-14 [Data: OCR MEI and the Standard Deviation]

[beatek]

hi jack, i just wanted to say thank you so much for your time and effort, your videos are a complete staple for me. i cant believe how efficient and motivated you are as a teacher. my a level teachers are so confusing to the point that you have essentially tought me my entire course, and despite the fact you teach aqa, you go out of your way to specify things relative to my board (ocr mei) which in combination with your valiant efforts on youtube, deserves the teacher of the century award... thank you truly

[hollywaller]

Thank you for this video.
I was hoping you could answer a specific MEI question I have. (Any advice would be appreciated!)

On the 2020 Pure and Stats paper from OCR MEI, I'm confused by part of Q8 and Q9.


They question gives you summary statistics for a sample size of 500 from a population of 4960.
This data includes s AND sigma.
I know from this video that MEI doesn't use sigma like other exam boards, but still confused.
As part of Q8d we have to find the probability that a person is over 65.
In the mark scheme they use s rather than sigma.
My question is: if we know sigma, why would we use s?
I thought the purpose of s was that we can't know sigma and s is a good approximation.

If I'm wrong and sigma is not the population std dev then what is? How do we know what it is?



Same sort of question for Q9. (hypothesis testing with a smaller sample)
the question gives you sigma and s.
The mark scheme uses s/sqrt(n) for the standard deviation rather than sigma/(sqrt(n)
But again why? What is the purpose of the sigma given in the question? Is there ever a case where we should use sigma rather than s?

[x_nova_x]

sir i do ocr mei b h640 one of the questions for my a level exam was calculating the variance and i divided through by n-1 so would i be correct?