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# Edexcel S3 Help!!! watch

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1. When are you meant to combine the cells, in the chi squared questions, also how do you work out the degree of fredoms??

for example the question is below:

what i did; d.o.f = 5-1-1 = 3

-1 for working out mean
-1 (as you always have to minus 1 that what i have been told )

h0= poissonly distributed
h1= not poissonly distributed

then i worked out sum of oi^2/ei -n = 106.255-100= 6.256

X^2(5%)(3) = 7.815

6.256 < 7.815

reject h1 as mangers claim is correct data is poissonly distributed

but ms says my d.o.f is wrong and my sum of oi^2/ei -n is also, as i have to combine the last two cells? why is this????

2. You should combine cells so that none of them are less than OR EQUAL to 5. I don't know why; but the textbook emphasizes on this bit at least.

You minus 1 for calculation of the mean and -1 because of the additive constraint. That the sum should add up to a certain value so the last cell or a single cell has no degree of freedom.
3. Jan 2006 Question 5

Why should you use n=80 for the girls here?

5. Upon entering a school, a random sample of eight girls and an independent random sample of eighty boys were given the same examination in mathematics. The girls and boys were then taught in separate classes. After one year, they were all given another common examination in mathematics.

The means and standard deviations of the boys’ and the girls’ marks are shown in the table.

Examination marks
Upon entry After 1 year
Mean Standard deviation Mean Standard deviation
Boys 50 12 59 6
Girls 53 12 62 6

You may assume that the test results are normally distributed.

(a) Test, at the 5% level of significance, whether or not the difference between the means of the boys’ and girls’ results was significant when they entered school.
(7)
(b) Test, at the 5% level of significance, whether or not the mean mark of the boys is significantly less than the mean mark of the girls in the ‘After 1 year’ examination.
(5)
(c) Interpret the results found in part (a) and part (b).
(1)
4. Maybe a typo. The y is missing.

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