Edexcel S1 - Explanatory vs Response Variables

zetafunc. · 2012-05-06 22:32:28

Hi, could someone help me understand how to identify these two? Here's a question from the Jan 2012 paper:

"The age, t years, and weight, w grams, of each of 10 coins were recorded. These data are
summarised below.

[data]

Identify, with a reason, which variable, t or w, is the explanatory variable."

They've said that it's 't', because age is set and weight varies.

But why is age set? Isn't that assuming they're all the same age, or that during the experiment they aren't aging?

It's a 2-mark question and it's really bugging me. How are you supposed to know which is which?

Bob · 2012-05-07 06:50:57

hi zetafunc

explanatory variable is what I would call (showing my age) the independent variable. In an experiment, it's the one you can choose; the other being the one you would measure.

If the question goes on to ask something like "is there evidence that coins are getting lighter?" then it is clear they are regarding time as the chosen variable and weight as the dependent variable.

I cannot download this paper ... too recent on the Edexcel site. Is it available elsewhere?

Have a look at

http://en.wikipedia.org/wiki/Dependent_ … _variables

Bob

ps. When are you going to become a member? advantages: you ( and I ) can search previous posts; you can see when someone is on-line; you can password protect your username which could be very useful to protect your other posts. disadvantages: your email and IP are known to moderators; otherwise none I can think of.

zetafunc. · 2012-05-07 20:14:49

Hi, thanks for the response. I downloaded the paper from (remove spaces): mathspapers . co . uk / edexcel.html

Bob · 2012-05-07 20:24:43

I'll check that now.

Bob

Bob · 2012-05-07 20:41:20

hi zetafunc,

That's three separate places that EdExcel hides its past papers. That one looks the easiest to use so I've saved the url.

Find the equation of the regression line of w on t in the form w = a + bt

That's the clue. There is a separate calculation to get a regression line for t = c + dw.

The one they've asked for implies they are treating t as the explanatory variable. ie you choose it and calculate w from the regression line.

In the mark scheme they say

The explanatory variable is the age of each coin. This is because the age is set and the weight varies. B1 B1

So you get one mark for saying 't' and another for the reason. They will allow for a variety of sentences here, so you don't have to use the word 'set' to get the mark.

eg. 't' is the chosen variable and 'w' calculated from it
The regression line calculates 'w' given 't'

etc etc

They are looking for mathematical understanding, not the exact same language as they used.

Bob

zetafunc. · 2012-05-07 20:51:16

Oh okay, thanks. So if they ask you to find the regression line of the form y = a + bx, then is x always the explanatory variable?

Thanks for telling me I didn't have to give that exact reason.

Also, another question, sorry -- do we have to know definitions for S1? I have heard rumours that the examiner changed in May 2007 so the paper style has been different since then. I have also noticed that there haven't been any questions that ask you to define what a sample space or an event is, or to explain the stages of using a statistical model.

I suppose I should be happy that S1 and D1 on the same day, two exams with about the same level of difficulty...

Bob · 2012-05-07 22:05:33

So if they ask you to find the regression line of the form y = a + bx, then is x always the explanatory variable?

imho YES.

Don't know about the definitions. Sorry.

New examiner shouldn't make any difference as it's the syllabus that dictates the questions. After a paper is set (usually about a year ahead!), it is independently checked to make sure it does the job ie. questions asked are on the syllabus, properly worded, all areas covered, timing reasonable, same area not tested excessively etc. So there shouldn't be any surprises.

This wasn't always the case. Years ago before GCSE, a question asked for a cumulative frequency curve; except it didn't! The examiner asked for an ogive. Hardly anyone, candidates and staff, had a clue that's what he meant. Different era, different words. Big stink, board had to scrap the question and base marks on the rest. Over the years things have got better and better in this respect.

If you see a diagram with the words "Diagram NOT accurately drawn here" that's one of my ideas. After we'd had candidates measuring when they were meant to use trig.

Bob

zetafunc. · 2012-05-08 21:21:01

Hmm, that makes sense. I've seen diagrams with that caption in my GCSE maths papers, it's true, a lot of my friends tried to measure angles with protractors.

It's okay, I will just memorise all the definitions that came up before.

Also, could you recommend any tips with regards to what exam to pick first in a clash? I heard that you're more likely to do better in the first exam, but is that true? My clashes are:

S1/D1

M2/D2

C4/S3

FP2/S4

FP3/M5

zetafunc. · 2012-05-10 06:28:10

Also, do you know if pencil marks will show up on the scanned photocopy? Pretty silly question but I'm planning on doing most of the D1 exam in pencil and then going over it in pen -- but this means I'll have to rub out the pencil marks. Will the rubbed out pencil marks show up clearly in the scan?

Bob · 2012-05-10 06:58:15

hi zetafunc,

Sorry, forgot to answer your other question.

Which to do first?

Your centre will allow you to choose?? Wow! Most places make you do the one that suits the invigilation requirements (If you've got to be kept 'incommunicado', it's easier if you are the only one etc...)

But if you really have a choice .... I'd go for the exam I find easier first so I don't get too worn out and agitated for the second tougher one. But that's my personal opinion ... you may feel differently.

pencil marks?

I suppose you are doing this because you make mistakes and want to correct them. I wouldn't do this at all. No time to re-write everything. If you want to alter something, just put a line through that bit AFTER you have written what you really want marked.* But to answer your question ... will the rubbed pencil show? ....... that will totally depend on how hard you press, what hardness of pencil you use and how well you rub it out. What you should be worrying about is "Can the marker read what I've written?" If your writing is untidy anyway and you've got traces of pencil too, that sounds like you are making your work less clear.

* The exam time is for doing the questions. Check this with your exams officer but I'm pretty sure you can do tidying up (eg. crossing out bits that you don't want marked) after time has been called to stop working. This is not part of the exam. So if you are really pushed for time you can squeeze another few seconds that way. Same goes for tying in any extra sheets and writing your name on them.

Fortunately, the markers want to give you the marks; they are not sadistic b******. They will struggle to make sense of your work and they can blow up the magnification to "zero-in" on tricky writing. Once I had a problem working out if a candidate had written x² or x³. It was written so badly that even at high mag. I couldn't tell. I referred it and my superviser said give the mark. I am not recommending that you deliberately write badly just to gain more marks. You don't need to do that because your maths is quite strong enough anyway!!

Bob

zetafunc. · 2012-05-10 07:33:33

Thanks, this was helpful.

Hmm, I was thinking of going for the easier exam first, too. It is difficult to decide even then, though.

Well, I am mostly worried about this for the D1 exam. There is a lot of drawing in that exam. For example, in critical path analysis, I have to scribble out my activity network repeatedly because it doesn't look neat. Speaking of which, do you know if in my activity network the lines are allowed to cross at all? Do the lines for the activities have to be straight lines, or can they be curved?

Regarding the tidying up after the allotted time has elapsed, how does this work? Would an invigilator come over to me and rub it out for me?

My writing is pretty bad, but that has eased my mind a bit knowing they aren't so sadistic! I always thought I'd never get full UMS in any exam because it is terrible.

anonimnystefy · 2012-05-10 08:26:02

Bob! I am very disappointed! B******?!

Bob · 2012-05-10 10:15:24

zetafunc:

Regarding the tidying up after the allotted time has elapsed, how does this work? Would an invigilator come over to me and rub it out for me?

I'm going to check with my friendly exams officer lady before saying any more in case I get this wrong. I'll try to post an answer tomorrow.

anonimnystefy: Why? Whatever you thought about this came out of your head.

Bob

zetafunc. · 2012-05-10 21:35:43

Thanks, that would be really great.

Do you have access to the D1 Jan 2002 paper by any chance? I'm stuck on Q7e...

Bob · 2012-05-10 21:49:16

http://www.mathspapers.co.uk/edexcel.html

only goes back to 2007.

Please post the whole question.

I've got to follow a link given to me about finishing up an exam. I'll post again shortly.

Bob

zetafunc. · 2012-05-10 21:55:07

The question paper is here (remove spaces):

w ww . thomas-reddington . co m / uploads / 7 / 7/ 8/ 3/ 778329/d 1_qp_jan_2002 . pdf

The answer booklet is here (remove spaces):

clc2 . uniservity . co m / GroupDownloadFile. asp? GroupId = 320323 & ResourceId = 1114722

In the mark scheme they have shifted the critical activity, F. I thought this was not allowed...

Bob · 2012-05-10 22:04:26

hi zetafunc

The following is taken from the jcq 'instructions to invigilators' document, page 35

http://www.jcq.org.uk/attachments/publi … 011-12.pdf

19. Finishing the examination
At the end of the examination invigilators must:
tell the candidates to stop working;
allow candidates who arrived late, and were allowed the full working time to do their examination, to continue after the normal finishing time and tell them to stop working after the extra time allowed has passed;
instruct candidates taking written examinations to: -
o make sure they have put all the necessary information on their scripts and any additional answer sheets, e.g. candidate name, candidate number, centre number;
o make sure their answers are correctly numbered;
o put any loose sheets in the order they answered the questions, and then fasten them and any supplementary answer sheets to the back of their answer booklets with a treasury tag. Paper clips or staples must not be used.

Note it does not say stop writing; it says stop working. And it is followed by the instruction to sort out scripts etc.

So, let's imagine a possible scenario:

A candidate has messed up a pre-printed graph sheet and re-done the question on separate graph paper. Then, during the last 5 minutes of the exam, he spots that part (b) of a four part question is wrong. So he does it again after part (d). He is just writing the last digit when candidates are told to stop working.

It is permissable to do all of the following:

(i) Ask for a treasury tag to fasten the loose script in.

(ii) Write name and candidate number on this graph sheet and fix it in the answer script.

(iii) Put a line through the incorrect graph and write "Done again on a separate sheet."

(iv) Put a line through the incorrect answer (b) and write "Done again after part (d)."

(v) Write "Corrected part (b)" at the start of the revised answer.

All these tasks are considered to be "admin" not "working".

Hope that clears that up.

Bob

Bob · 2012-05-10 23:02:19

D1 Jan 2002 paper by any chance? I'm stuck on Q7e...

It seems to me you cannot achieve 21 days with only two workers.

If worker 1 does the CP (A C F and H) and worker 2 does say B D and G then who does E?

So have you been taught a method for finding the minimum that satisfies the constraint on workers?

I don't remember that it is on the syllabus.

I suspect they just want you to play around with the tasks so as to find the best job assignment. (best = minimum time)

A can be disregarded for this.

The total time for the rest is 37 days, so the optimum is going to be 18 and 19.

So play with the arithmetic until you find an assignment of jobs that gives these and you know it is best (provided the Gannt shows the dependencies work).

The MS says "eg." so maybe there's another answer that also works. But I'd have expected the MS to show it, otherwise the poor marker has got to check every answer that isn't the one shown.

I've no idea how a marker would assign marks if a candidate offers a reasonable, but not best, solution. I cannot believe they would give zero! But it was 2002. Quality of questions has improved so let's hope they don't set one of these for 2012.

LATER EDIT:

I've found 4 numerical solutions: BCE-DFGH BFG-CDEH BCG- DEFH and BEG-CDFH.

But only the last will work with the dependencies. 'A' may be put with either worker.

Bob

Bob · 2012-05-11 00:29:17

further comment:

The syllabus just says "scheduling"; which is vague to say the least.

But compare with the 2010 and 2011 questions.

The questions are almost identical except for the final part. The recent papers are much more specific and straight forward aboit what they want in regard to scheduling. Reading between the lines I'd say that 2002 question was found to be unsatisfactory at the time. Possibly very few candidates managed to come up with that answer even amongst the stronger students. As a result they've become more careful about what they ask.

post #7

After a paper is set (usually about a year ahead!), it is independently checked to make sure it does the job ie. questions asked are on the syllabus, properly worded, all areas covered, timing reasonable, same area not tested excessively etc. So there shouldn't be any surprises.

imho the 2002 question fails the 'on the syllabus' and 'timing reasonable' tests.

Bob

zetafunc. · 2012-05-11 21:10:56

bob bundy wrote:

The following is taken from the jcq 'instructions to invigilators' document, page 35
Note it does not say stop writing; it says stop working. And it is followed by the instruction to sort out scripts etc.
So, let's imagine a possible scenario:
A candidate has messed up a pre-printed graph sheet and re-done the question on separate graph paper. Then, during the last 5 minutes of the exam, he spots that part (b) of a four part question is wrong. So he does it again after part (d). He is just writing the last digit when candidates are told to stop working.
It is permissable to do all of the following:
(i) Ask for a treasury tag to fasten the loose script in.
(ii) Write name and candidate number on this graph sheet and fix it in the answer script.
(iii) Put a line through the incorrect graph and write "Done again on a separate sheet."
(iv) Put a line through the incorrect answer (b) and write "Done again after part (d)."
(v) Write "Corrected part (b)" at the start of the revised answer.
All these tasks are considered to be "admin" not "working".
Hope that clears that up.
Bob

Thanks for getting the direct quote from JCQ. I usually have to ask for extra paper in my exams (not maths) and usually they have told me to worried about attaching them after the allotted time has elapsed. I didn't know you could do all of those things though, thanks for letting me know. I may have to do (iii) a few times in the D1 exam!

zetafunc. · 2012-05-11 21:14:28

bob bundy wrote:

D1 Jan 2002 paper by any chance? I'm stuck on Q7e...
It seems to me you cannot achieve 21 days with only two workers.
If worker 1 does the CP (A C F and H) and worker 2 does say B D and G then who does E?
So have you been taught a method for finding the minimum that satisfies the constraint on workers?
I don't remember that it is on the syllabus.
I suspect they just want you to play around with the tasks so as to find the best job assignment. (best = minimum time)
A can be disregarded for this.
The total time for the rest is 37 days, so the optimum is going to be 18 and 19.
So play with the arithmetic until you find an assignment of jobs that gives these and you know it is best (provided the Gannt shows the dependencies work).
The MS says "eg." so maybe there's another answer that also works. But I'd have expected the MS to show it, otherwise the poor marker has got to check every answer that isn't the one shown.
I've no idea how a marker would assign marks if a candidate offers a reasonable, but not best, solution. I cannot believe they would give zero! But it was 2002. Quality of questions has improved so let's hope they don't set one of these for 2012.
LATER EDIT:
I've found 4 numerical solutions: BCE-DFGH BFG-CDEH BCG- DEFH and BEG-CDFH.
But only the last will work with the dependencies. 'A' may be put with either worker.
Bob

Thanks for taking the time to look through the paper. So it is sort of trial and error from that point? I know what you mean -- the questions before the new spec came out in 2008 were a lot trickier and some of them just shatter my confidence for the exam.

If you don't mind me asking, how long did it take you to find those 4 numerical solutions, and was there a general technique you used? I suppose it's like the route inspection, you just need to practice... but you are sure this kind of question will not come up again?

zetafunc. · 2012-05-11 21:19:51

bob bundy wrote:

further comment:
The syllabus just says "scheduling"; which is vague to say the least.
But compare with the 2010 and 2011 questions.
The questions are almost identical except for the final part. The recent papers are much more specific and straight forward aboit what they want in regard to scheduling. Reading between the lines I'd say that 2002 question was found to be unsatisfactory at the time. Possibly very few candidates managed to come up with that answer even amongst the stronger students. As a result they've become more careful about what they ask.
post #7
After a paper is set (usually about a year ahead!), it is independently checked to make sure it does the job ie. questions asked are on the syllabus, properly worded, all areas covered, timing reasonable, same area not tested excessively etc. So there shouldn't be any surprises.
imho the 2002 question fails the 'on the syllabus' and 'timing reasonable' tests.
Bob

So do you think it is worth it doing these older past papers? I have racked my brain on some of them trying to get a solution, and to me quite a few of them just plain don't make sense!

Take this one for instance:

desmond.imageshack.us/Himg851/scaled.php?server=851&filename=d1mistake.jpg&res=landing

"(a) Use each of these two alternating paths, in turn, to write down the complete
matchings they generate."

I got:

A-3
B-2
C-1
D-5
E-4

and

A-2
B-4
C-1
D-3
E-5

But the mark scheme says:

C-1
B-2
B-3
E-4
D-5

and

C-1
A-2
D-3
B-4
A-5

This too is from an older paper. I never have this problem in papers from 2009 onwards. Is it just a case of there being multiple solutions and they haven't written them down?

Bob · 2012-05-12 01:01:11

hi zetafunc,

I agree with your answers.

But the mark scheme says:
C-1
B-2
B-3
E-4
D-5
and
C-1
A-2
D-3
B-4
A-5

These just look like typos to me as letters are repeated / others missing.

On the more general point of doing past papers.

The syllabus did change ( 2007 ?) . It doesn't seem to have made much difference in the questions but it may account for some of the difficulties you are having. I'd have to look carefully at the before and after, but I think they moved some topics about a bit.

Do you really need to do every paper that exists?

If you are getting >90% on a practice then you can say that module is fairly safe and concentrate on the other modules. And if you find a topic that is more difficult, then the older papers might be useful to 'dip into'; just trying the questions you want to get better at.

When you do a past paper do you create 'exam conditions'; no coffee, radio, phone off, etc. work strictly to time, etc ?

Bob

zetafunc. · 2012-05-12 01:09:50

Ah okay good, I was worrying about it last night. Thanks for putting my mind at rest.

Yes, they moved some topics around; simplex has been put into D2 (formerly D1) and flow networks (finding augmenting routes, that kind of thing) has also been put into D2. Unfortunately what that has done is made D2 a really tedious module, and even longer than FP3!

Well, I don't know, it's just what works for me. I do every paper that exists so I can be even more confident about the exam -- if I've seen pretty much every possible combination of every question that could come up, I should have a decent chance at getting >90%.

I used to create exam conditions, but now I just work in front of my computer and do past papers question by question, sometimes listening to music (classical to make me relax). Before when I did a whole paper under exam conditions and marked it at the end, I sometimes felt less bothered to go back and re-do whole questions, especially if there were lots of mistakes. This worked well and was evident from my results:

Year 11 (old revision methods):

C1: 100
C2: 89
C3: 92
M1 : 88

Year 12 (new revision methods):

S2: 98
FP1: 100

Although it is a small sample size, I think it has had an effect. Although, you may also have to take into account that in Year 11 I had to do GCSEs at the same time (and took them early so I was more stupid than I am currently). I just hope those results are not anomalous and I end up getting 70-80 in my June modules

zetafunc. · 2012-05-12 01:12:55

Another concern I have is how to manage my revision for all the modules I have. I'm quite confident about D1 and S1 now which is good because those exams are only 6 days away, but D2 is 19 days away and I haven't properly started learning the content. I know that I could cover all the content in a day if I just worked at it constantly, but is this wise given that I have some exams that are closer?

And then there is M4 in 20 days which I'm really dreading...

Math Is Fun Forum

#1 2012-05-06 22:32:28

Edexcel S1 - Explanatory vs Response Variables

#2 2012-05-07 06:50:57

Re: Edexcel S1 - Explanatory vs Response Variables

#3 2012-05-07 20:14:49

Re: Edexcel S1 - Explanatory vs Response Variables

#4 2012-05-07 20:24:43

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#5 2012-05-07 20:41:20

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#6 2012-05-07 20:51:16

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#7 2012-05-07 22:05:33

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#8 2012-05-08 21:21:01

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#9 2012-05-10 06:28:10

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#10 2012-05-10 06:58:15

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#11 2012-05-10 07:33:33

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#12 2012-05-10 08:26:02

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#13 2012-05-10 10:15:24

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#14 2012-05-10 21:35:43

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#15 2012-05-10 21:49:16

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#16 2012-05-10 21:55:07

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#17 2012-05-10 22:04:26

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#18 2012-05-10 23:02:19

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#19 2012-05-11 00:29:17

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#20 2012-05-11 21:10:56

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#21 2012-05-11 21:14:28

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#22 2012-05-11 21:19:51

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#23 2012-05-12 01:01:11

Re: Edexcel S1 - Explanatory vs Response Variables

#24 2012-05-12 01:09:50

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#25 2012-05-12 01:12:55

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