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Edexcel Stats 3 need help with one question!! (Normal distribution tests)

http://pmt.physicsandmathstutor.com/download/Maths/A-level/S3/Papers-Edexcel/June%202013%20QP%20-%20S3%20Edexcel.pdf

June 2013- Question 4 on normal distrubtion test. it is a really weird question, as it does the table for you, but it uses the full values such as 3-5, 5-6.

The part im stuck on is part b, find the value of 'b' which is the value of x > 8

I simply used the value of p(x<8) that i used to find the value of 'a' and did 1-p(x<8) and then i multiplied by 50 to get 8.425, however the answer that it apparently should be is 8.34. With the value 8.34 the test statistic will be correct, but for my value of 8.425 the test statistic is not in the range. Anyone can see where I went wrong? I followed the method from the book.

http://pmt.physicsandmathstutor.com/download/Maths/A-level/S3/Papers-Edexcel/June%202013%20MS%20-%20S3%20Edexcel.pdf

Reply 1

Unusualbob1

From what I understand, one of the degrees of freedoms is used up as a result of the fact that the expected frequencies must total 50. In effect I think this means that the last cell is fixed based upon your value of a. So you should be doing 50 - (All the expected frequencies added together). If you do it with the probabilities as you have done, I think you introduce a small amount of error from the table values. If you use 8.425, your total expected frequencies row will exceed 50.

Reply 2

Unusualbob1

Also does anyone know if they could ask you to do the full goodness of fit test for the normal distribution? This is the only past paper question I have seen so far on it. I find it more difficult than the other tests as they are more things you have to think about.

Reply 3

hascho9

Original post by Unusualbob1

What do you mean by a full goodness of test? do you mean having to do the second table yourself? In that case I do think they can ask it, and as far as I have seen it hasnt come up yet. Maybe a topic to practise before tommorow!

Also do you happen to know for sure how many significant figures to give your answers in? mark schemes are confusing me, but it seems most of the time it should be 3 sig figs unless stated?

Reply 4

hascho9

Original post by Unusualbob1

Understood, thank you very much for clearing that up, i didnt realise it imposed that 'condition' on you so literally.

Reply 5

Unusualbob1

Original post by hascho9

Yeah I am going to go over that and the model answers for the different types of sampling. I have no idea about the significant figures but I think 3 should be ok as it will normally have the mark for "answer which rounds to". I seem to find the mark scheme figures are the most inconsistent for the (O-E)^2/E column.

Reply 6

hascho9

Original post by Unusualbob1

From what i understand, they give a range of answers that you can get for the O-E)^2/E column and the final test statistic. But yeah as far as I know keep everything to max figures until the final answer which should be 3 sig figs.

Also with the sampling, is the model answers a online resource! if so could you send it to me aswell, i was planning to write some revision cards and memorise them before the exam. Honestly one of my worst areas, even though its litteraly gcse stats.

Reply 7

Unusualbob1

Original post by hascho9

Okay thanks, I'm going to for 3.s.f unless stated otherwise then and more if used in a future calculation.

Have a look at the sampling section of this doccument - http://www.drfrostmaths.com/uploads/JFrost/files/S3CheatSheet.pdf
Yeah I have flashcards for that section.

Reply 8

hascho9

Original post by Unusualbob1

Thanks for this!

Also just to be clear, only round to 3 sig figs for the absolute final answer!

Reply 9

hascho9

Original post by Unusualbob1

One thing the cheatsheet is missing out on is the adv and disadv of census' and sampling in general.

also primary and secondary data adv and disadv, cant hurt to have a flashcard on them/