Edexcel Decision Maths 1 (General Help)

zetafunc. · 2012-04-15 00:23:32

I'm sitting the D1 exam in about 33 days and I have some worries about how to score all the marks in a question. I can apply all the algorithms and can get the right answer for every question they ask, but what they do in the mark schemes is strange to me. I would appreciate it if anyone who has looked at Edexcel D1 before could help me understand how to get the marks.

For example, D1 Jan 2009, the first question is a simple question asking you to sort 8 letters in alphabetical order using a quick-sort. They have not specified a starting pivot so I used the middle letter (5th position). What they've done in the mark scheme is drawn all the pivots in, but for the final pivot they've not indicated their pivot (is this because the algorithm is over?) and they've used highlighting for the temporary pivots! You are only allowed black/blue pen in the exam. Are they expecting me to draw a key to identify the temporary and final pivots (e.g. underlined = temporary pivot, boxed = final pivot)? They've not done anything of the sort in the mark scheme.

My second question is about critical path analysis; in the same paper (Q3) they've done the same thing I've done EXCEPT I drew the dummy from H to G rather than G to H. I don't understand why it matters; they haven't said anything about this in the mark scheme either. Is this allowed?

This is more of a general question... how the heck do you solve route inspection problems quickly? Despite only having 4 odd valences it just takes me ages to find a path that I can use. Sometimes I check by using Dijkstra but I can't do that in the exam because that takes far too long. Is there a general method to do them? I know you list the odd valences and identify which ones have to be traversed twice, but when you have something like 12 arcs I just get confused and trial and error takes forever.

Dijkstra's algorithm -- do you actually get marks for writing your working values? If so, how should you 'cross them out'?

And finally, for linear programming, when finding the feasible region, they want you to use shading and they give you 1 mark for this in the mark scheme. Does this mean shading the feasible region itself, or everything that is NOT constrained by the given inequalities?

Sorry to lump this all into one topic, the concepts in this module are very straightforward, it's just that the mark scheme is so... ugh...

zetafunc. · 2012-04-15 00:27:15

Also, when applying Kruskal's algorithm, is part of the algorithm recognising that when you have (n-1) edges for a graph with n nodes that adding any more results in a cycle? Will you lose marks if you continue to reject the remaining arcs (after you've found the minimum spanning tree)?

zetafunc. · 2012-04-15 03:37:24

Okay, why have they changed their mark scheme style in June 2009? Now you have to write all your temporary pivots in a separate table? What is this?? Can someone clarify how you're meant to answer these sorting questions?

Bob · 2012-04-15 04:18:31

hi zetfunc

Have you got this?

http://www.edexcel.com/migrationdocumen … 090312.pdf

The markers are required to award marks for correct methods leading to the required answer. Sometimes a candidate will find a different way to do a question. if the question says do X, and you do Y, then you won't get the marks but where either X or Y is possible the marker has to follow through your method; even if it's a pain 'cos it's not on his crib sheet. If he is unsure he has to refer it up to his supervisor who gives a ruling. They won't penalise you for doing it differently if your way also answers the question.

What they've done in the mark scheme is drawn all the pivots in

I think they are just trying to be economical with space.

In the past I've marked papers where one candidate does a whole question in half a page and another takes three pages, with both getting full marks.

(is this because the algorithm is over?)

Strictly you don't know this until you've tried it, so you wouldn't be wrong to have one more step.

They've used highlighting for the temporary pivots! You are only allowed black/blue pen in the exam.

The markers see a scanned, digital copy of your work so highlighting (whether allowed or not) might show up poorly.
What's to stop you writing beside your answer pivot = K or whatever.

I drew the dummy from H to G rather than G to H

I don't think it matters. The markers are meant to know that either is Ok.

how should you 'cross them out'?

Neatly so the marker can still read what is underneath.

Does this mean shading the feasible region itself, or everything that is NOT constrained by the given inequalities?

Looks to me like they've shaded the not included regions, (which is the better way because it leaves the allowed region uncluttered) but why take the risk; say what you're shading eg. "I am shading out the regions that are excluded by the constraints." That way you have done the sensible thing, and made it clear; you get the marks.

how the heck do you solve route inspection problems quickly?

Sorry, don't know this one. I suggest you do the routine questions first and leave any that might consume a lot of time until last. Get as many marks as you can quickly, then you've got time to tackle the indefinite bits. They shouldn't be setting any question where the time required is excessive anyway. If they do, complain to your teacher straight away so the board has to look at it and make allowances. It's rare but they have been known to credit for a whole question where they messed this up. ie. They gave up to full marks for the total of the other questions. They can also fiddle the final mark by UMS.

Bob

zetafunc. · 2012-04-15 06:21:54

The markers are required to award marks for correct methods leading to the required answer. Sometimes a candidate will find a different way to do a question. if the question says do X, and you do Y, then you won't get the marks but where either X or Y is possible the marker has to follow through your method; even if it's a pain 'cos it's not on his crib sheet. If he is unsure he has to refer it up to his supervisor who gives a ruling. They won't penalise you for doing it differently if your way also answers the question.

Hmm, I did not know this. Is this always true? For example, suppose I saw a differential equation in an FP2 paper that simply said 'solve'. If I did something like use a Laplace transform but still got the correct final answer (not that I would use that method), would it still be possible to gain all the marks? How would they start awarding method marks, if, for example, there was a 10-mark question in a paper that could be solved in, say, 2 lines with a much more efficient method, is it possible to score all the marks?

What they've done in the mark scheme is drawn all the pivots in
I think they are just trying to be economical with space.
In the past I've marked papers where one candidate does a whole question in half a page and another takes three pages, with both getting full marks.

Why is it that they've taken some pivots out on the last line though? I don't understand... is that part of the algorithm?

(is this because the algorithm is over?)
Strictly you don't know this until you've tried it, so you wouldn't be wrong to have one more step.

Ah okay, thanks. So I would not lose a mark for indicating my final pivots? I thought that afterwards you should finish with everything being a pivot.

They've used highlighting for the temporary pivots! You are only allowed black/blue pen in the exam.
The markers see a scanned, digital copy of your work so highlighting (whether allowed or not) might show up poorly.
What's to stop you writing beside your answer pivot = K or whatever.

So keys are allowed?

I drew the dummy from H to G rather than G to H
I don't think it matters. The markers are meant to know that either is Ok.

Okay, thanks.

how should you 'cross them out'?
Neatly so the marker can still read what is underneath.

Hmm... okay. What if I don't cross them out at all?

Does this mean shading the feasible region itself, or everything that is NOT constrained by the given inequalities?
Looks to me like they've shaded the not included regions, (which is the better way because it leaves the allowed region uncluttered) but why take the risk; say what you're shading eg. "I am shading out the regions that are excluded by the constraints." That way you have done the sensible thing, and made it clear; you get the marks.

Hmm, alright. So do they have to read everything that you write on your paper by law?

how the heck do you solve route inspection problems quickly?
Sorry, don't know this one. I suggest you do the routine questions first and leave any that might consume a lot of time until last. Get as many marks as you can quickly, then you've got time to tackle the indefinite bits. They shouldn't be setting any question where the time required is excessive anyway. If they do, complain to your teacher straight away so the board has to look at it and make allowances. It's rare but they have been known to credit for a whole question where they messed this up. ie. They gave up to full marks for the total of the other questions. They can also fiddle the final mark by UMS.
Bob

Okay, that makes sense. Thanks for taking the time to answer all my questions, I appreciate it. There are no D1/D2 teachers at my school unfortunately. I am a bit worried about this paper because of the high boundaries.

Bob · 2012-04-15 07:02:58

Using a different method from the expected one.

Well it's up to the person who sets the paper to make it clear isn't it.

Generally the exam board don't want you 'going off on your own way' with a question for precisely this reason.
It's horrid to mark.

So they try to write the question so as to steer you towards their way. But, if they really don't say, then yes, you can use any valid method. I might be a bit unhappy with trial and error, or a graph, but 'proper maths' that works has got to be OK.

In practice, I don't think it is going to happen.

pivots.

The mark scheme is just the markers crib sheet to show what sort of answer to expect and give marks for. They have to be flexible when they have a candidate's answer in front of them. It sound like you know what you are doing so don't get anxious about this.

keys allowed.

Yes, because your job is to show the marker that you can do the work. You can say anything that helps him to understand you. Just don't say " Here's a tenner' to 'see' I've got the right answer!"

What if you don't cross them out? Have you made your method clear? Ask yourself this for any question. If YES, that's OK isn't it. As a marker what I don't want is a load of untidy, incomprehensible nonsense, even if the right answer somehow 'pops' out at the end. (and once, in the middle of the working. What was I to make of that? Was the candidate offering it as the answer or was it just a number amongst the rubbish that happened to be the answer by a lucky coincidence!)

Do they have to read everything by law? Well it's not a law exactly, but yes, they have to read it all*. When I did marking for C2 I even had to look at the blank pages in the candidates' scripts, just in case something was written there. This was very annoying. The papers get transmitted to the marker electronically so you see each page on your computer screen. Quite often a candidate had got full marks on a question on the first of three pages. The software made me download the other two pages just in case. So it took three times as long as it needed to mark that question. gggrrr!

*But once a candidate has reached the correct answer we were told to read but disregard anything afterwards.

If you have to change an answer close to the end of the time, write the correction BEFORE you cross out the wrong version. Then if time is called before you cross, they will read the right bit and give you the marks. If you cross first and run out of time to write the correction they have nothing to award marks.

Good luck with the exam,

Bob

zetafunc. · 2012-04-15 07:36:23

Thanks for the reply. It is just that sometimes I wonder where my marks go... for example, I got a 98 in this January's S2 and I was sure that I got 100 (not that I really care, I am over the moon with 98), and according to Edexcel ResultsPlus I lost two marks on two different hypothesis questions for the same thing. When I looked at what mark it was, it was silly... they said that you got a mark for stating the critical region which they wrote as [0,3] instead of X ≤ 3. I wrote the latter and did not get the mark, though the former has never appeared in an S2 mark scheme before.

Another worry I had was about the definitions for D1 and D2... these are given in the specification. Am I supposed to memorise all of them? Some of them look quite formulaic and I've seen people make include their own 'lists' where they ignore some of them (e.g. the definition for 'total float').

Thanks for telling me about using a key, I guess now it is just up to me to point the examiner towards my answers.

I was also wondering if you could debunk a myth -- is it true that if you write multiple answers to a question (e.g. a 5-marker on Newton-Raphson method), each of them separate from each other, the examiner is forced to mark the highest-scoring one?

I wish I had known how flexible the mark scheme was before. I crossed out everything and then time was called in my C2 exam. I got 89 but I was not happy as that took a chunk out of my marks.

zetafunc. · 2012-04-15 07:50:49

Also, Q2, D1 Jun 2011... they ask for the definition of a Minimum Spanning Tree. The official definition in their glossary does not match up to the one in their mark scheme.

Question: 2) (a) (ii) Define the term "minimum spanning tree".

Glossary definition: A minimum spanning tree (MST) is a spanning tree such that the total length of its arcs is as small as
possible. (MST is sometimes called a minimum connector.)

Jun 2011 MS definition: A minimum spanning tree is a tree that contains all vertices and the total length of its arcs (weight of tree) is as small as possible.

If this happened in one of my papers, would I be able to complain about this and possibly get my mark raised? According to the mark scheme you had to mention the bit about them containing all vertices to get one of the marks. Why are they not using an identical definition to their glossary definition? Is there a newer glossary I should know about? The definition above (glossary definition) is from Edexcel's latest specification according to their website.

zetafunc. · 2012-04-15 07:55:05

Sorry, ignore the above post. I just did some research and found out they lowered the 100 UMS boundary to 74/75 because of this definition question.

Bob · 2012-04-15 08:01:24

You are beginning to ask questions that I do not know.

The decision syllabus must have changed since I taught it as some topics you mentioned weren't not on it then. So I don't know about memorising formulas. That wouldn't suit me as I am hopeless at remembering anything. If you look at my other posts you'll see I tend to start from first principles rather than trotting out a formula. There are lots of past papers available so that should give you a good idea what you have to know.

I also don't know about the multiple answers issue. I think they mark the first. Tricky.

The reason markers have to look at everything is more complicated.

As you know the answers go on a standard paper which has a bar-code on every page to identify the candidate.

When the papers are received at the board a machine dissects them and the pages are scanned. Markers get a batch of, say, question 1s to mark. Sometimes a candidate writes the answer to another question in that space. They are told not to, but, the exam board had decided not to penalise someone because of that. So if a marker discovers the wrong answer what is he to do? He cannot mark it, because the software is accepting marks for a different question. He has to refer the question back to the supervisor who gets the actual paper and hand marks it. That's why I had to look at blank pages, just in case they weren't blank. I am assuming that this is still the case; it was several years ago.

Bob

zetafunc. · 2012-04-15 09:13:00

The D1 syllabus changed in 2008 I think (the first new spec D1 paper was January 2009). Basically they moved some D1 stuff into D2 and put some new stuff into D1.

I am surprised that they can let you do one question in the wrong place. I thought you wouldn't get marks for that. I am glad that they do that though.

Bob · 2012-04-15 10:10:34

Well they did when I marked but obviously don't rely on it!!!

Bob

zetafunc. · 2012-04-15 22:59:34

That's okay, I hope that will never happen!

I have another question about this exam (well, D1 and D2). In the exam you get an answer booklet. However, it's quite easy to make a mistake on one of the diagrams and even though you were so sure you were correct, you spot that it's wrong and you have done it in pen. What do you do in this situation? Can you ask for a new answer booklet? Are the answer booklet and question paper separate or in the same booklet?

This sort of applies for other exam papers too, like S1. What happens if you really mess up the graph but you've done it in pen so it is hard for the examiner to decipher what you have done? Will I just have to be extra-careful to make sure that doesn't happen?

Bob · 2012-04-15 23:20:49

hi

If these papers are like the ones I marked (and I'm pretty sure they are) then the answer paper you get given is 'personalised' to you. It'll have your name on it and every side of paper has a bar code that identifiies it as your work. Then when it is split up and scanned at the board, the computer can keep track of every page of your work.

So I don't think you can start a new booklet.

So what should you do?

(i) If you have space, do a correction, and only then, cross out the bit that you don't want marked. That should cover minor errors but is the preferred thing to do because the alternative is complicated for everybody. Best of all, don't make mistakes!

(ii) If it's onto a specialised, and pre-printed, diagram like a graph or a partly completed CPA, then obviously you won't be able to do that, if you haven't got space. So you stick up your hand and let the invigilators advise. They should have supplementary pages for this situation. You re-do the question and tie that page into your script.

But now you want to make sure the marker is aware that a page exists that isn't bar coded. So write on the bar coded page for that question what you have done. "This answer comtinued on a separate sheet. " The marker will not be able to mark your question because they will have no way to associate your unseen work with the bit that has your bar code.

What they have to do is refer your work back to the supervisor. That person will be at the board and will get your complete paper out of the files and hand mark it. (Probably only that question as the rest can be assessed as usual.)

I'm retired now, but I'm going into school later today to talk to some colleagues, so I'll check this with the lady who does the exams. I'll check about supplementary sheets and how the school is supposed to handle them. So watch for a further post later this evening.

Bob

zetafunc. · 2012-04-15 23:52:12

Thanks for your reply. I am glad to know that they should have supplementary pages for pre-printed stuff, as in past papers I've been frequently messing up cascade (Gantt) charts and CPAs. I guess I just have to make everything as clear as possible to the examiner... someone once advised me to write all words in capitals too. I have lost marks in the past because of illegibility.

Do you happen to know how often mistakes occur in marking? Are they rare?

Bob · 2012-04-16 00:00:04

I'm assuming they have supplementary pages. Hope to find out for sure this afternoon.

I have lost marks in the past because of illegibility.

Do you have a recognised special needs in this respect? Another forum member has dyslexia and I'm hoping to arrange extra arrangements to allow her to show her best performance. That's the main reason I'm going into school today; to talk to the learning support staff.

You could go to www.jqc.org.uk and look for their access arrangements document.

Better go now or I'll miss lunchtime at school.

Bob

zetafunc. · 2012-04-16 00:03:13

I don't have any recognised special needs, no... I just have pretty bad handwriting. I rush and write too quickly and it looks like a scrawl ("doctor handwriting" as people say). I can write with individual letters, but I lose a lot of time that way. I think that is why I do better in maths exams, there is less writing. I'd like to have one of those laptop things but simply having messy writing is not an excuse according to my school.

Okay, see you later.

Bob · 2012-04-16 03:13:10

hi zetafunc,

The AQA special support lady said that a word processor (no other features) is Ok for anyone without a special application. Don't your school want you to get the best marks possible?

Who did you talk to? it might be worth asking again. But I think you may have to write as well (not sure how it works). But I sympathise. My writing is pretty unreadable when I'm up to speed too.

A paper can also be 'transcribed'. ie. A person re-writes what you have written and both copies are sent. Rather bizarrely, this person is not allowed to ask you to say what you have written. Eh? The final option is to have a scribe who writes as you dictate but that's very difficult to set up. You have to have lots of paperwork to support the claim and the school has to provide a separate room with a scribe and am invigilator independently. That seems very unlikely for you.

I spoke to 'my' school's exams lady. She said the board ask her to provide graph paper and other paper in case of need. Candidates just ask for it and it gets tied in with the script. She was pretty certain that your school would be on the scanned paper system because the whole of the UK is now.

Bob

zetafunc. · 2012-04-16 07:23:43

I talked to the head invigilator and she said it was just for people who suffered from dyslexia. I will ask them again.

Hmm, I am guessing it is because you could easily lie about what you wrote and if you had thought of the correct answer at that moment you could say that instead... I am not sure about the dictating thing, I have heard about it and it might take a bit long to do it (I may end up running out of time even faster).

Okay, that's good that they have spare graph paper if necessary. I will still use a pencil just in case though (and then go over it in pen at the end).

Thanks for taking the time to find out about this!

Bob · 2012-04-16 08:27:42

You are welcome.

Best of luck (not that you should need it ) in your exams.

Bob

zetafunc. · 2012-04-17 07:02:58

Thanks! I think I will need it to be honest, it is so hard having so many exams... for me anyway.

Another question I wanted to ask and my teacher didn't know the answer -- I'm a little worried about how to round my answers in exams. I've always used the general rule of 3 significant figures in my exams, but if they don't tell you what to round it to, would it be okay to give your answer to both 3 s.f. and 2 s.f.? In other words, to write both? Or is that not allowed?

This is mostly an issue in mechanics and statistics. In S3/S4 for some reason sometimes they even require 4 d.p. or 5 s.f., and I don't understand when to do that if they don't specify.

Bob · 2012-04-17 07:45:54

hi

Rounding should be 'sensible'. If the values at the start of a problem are, say, 3 sf, then your answer cannot be more accurate than the same, and may well be less.

To hedge my bets, I'd give a answer with, say 6 places, and then round it. They would expect you to use your common sense and would probably think an answer with less figures shows you've thought about it more than someone who just writes down every figure from their calculator indiscriminately.

But, stop worrying about it. If they haven't specified, then they must accept a range of possibilities, mustn't they.

I think we were told things like accept 4.45(anything after this) or accept anything that rounds to 4.5

LATER EDIT:

Here's an interesting exercise which I give to my year 9 pupils.

"I've measured a rectangle and, to the nearest centimetre, got 6cm by 4cm. What's its area?"

Of course, I get a chorus of 24 square centimetres.

"OK, think again. I said to the nearest centimetre. What's the smallest measurements that would round up to 6 by 4?"

5.5 x 3.5

"And the largest?"

6.5 x 4.5

Well 5.5 x 3.5 = 19.25 and 6.5 x 4.5 = 29.25 ... so the true answer could be anywhere from 19.25 to 29.25.

That 24 is beginning to look like over optimistic accuracy. I cannot even be sure of the 20.

Before the days of calculators students had to round off quite a bit because log tables and slide rules wouldn't give you more than a few places of accuracy. Now we can easily see an answer in the display with 8 or 10 figures. That doesn't mean we 'know' the answer that accurately!

Here's a simple one to ponder.

For some obscure reason I want to know the diameter of a tree. They won't let me drill a hole through so I'm doing it by measuring the circumference and then using pi to calculate an answer. I've got C = 110 cm. What would you give as the diameter?

Bob

zetafunc. · 2012-04-18 04:42:35

Hmm, thanks for the reply. It is just that sometimes when I look at some mark schemes they don't have anything that says AWRT (answers which round to) sometimes they just state the answer with an A1 mark beside it.

I agree, if I saw a question with each figure given to 2 s.f. I'd use 2 s.f. for my values too. But in Edexcel Physics I see them ask questions where their values are given to 1 s.f. yet they expect you to use g = 9.81.

Regarding your tree question, that 110 cm looks a bit like a trap. That is to 2 s.f.; but would someone writing the exams expect your answer to be to 2 s.f.? They could have written 110.00. If this were an exam question I'd use my calculator button for pi. In reality I suppose it is better to use pi = 3.14 but since your answer is to 2 s.f. then pi = 3.1 may be better.

zetafunc. · 2012-04-18 05:31:02

Sorry to be a pain, but I have a question about Q5a in D1 Jan 2010... it's an algorithm question. What determines whether or not they start a new line? Why after the first decision box do they begin starting new lines for each step?

Bob · 2012-04-18 06:01:13

No, use your calculator button by all means. But remember your answer will only be as good as the poorest known value; so 2sf is the best you can say.

And there are other reasons that put the accuracy in doubt.

(i) Where was this circumference measured? Trees usually get narrower as you go up. Maybe that reason for wanting to know the diameter shouldn't be 'obscure'.

(ii) And how circular is a tree anyway?

(iii) And bark is bumpy and fissured so the exact start and end point of the measurement is in doubt.

I expect that's why the physic's answers are given to a lower accuracy than you expected. Physicists live in the real world.

One exercise that I have given to my classes was to draw any triangle, measure its angles and add them up.

For a while I would circulate, sorting out those who couldn't use a protractor properly, but eventually I'd be at the point of asking what totals they had got.

I usually got answers ranging from 178 to 182 degrees.

OK, what conclusion would you make?

One lad said "That the angles of a triangle add up to about 180°."

It was the first time any pupil had included the word 'about'.

But I had to admit, based on the experiment, it was an excellent conclusion! What is absolute accuracy anyway?

I leave you to ponder this to whatever extent you like. There is no expected answer so it is not necessary to post one.

Bob

Math Is Fun Forum

#1 2012-04-15 00:23:32

Edexcel Decision Maths 1 (General Help)

#2 2012-04-15 00:27:15

Re: Edexcel Decision Maths 1 (General Help)

#3 2012-04-15 03:37:24

Re: Edexcel Decision Maths 1 (General Help)

#4 2012-04-15 04:18:31

Re: Edexcel Decision Maths 1 (General Help)

#5 2012-04-15 06:21:54

Re: Edexcel Decision Maths 1 (General Help)

#6 2012-04-15 07:02:58

Re: Edexcel Decision Maths 1 (General Help)

#7 2012-04-15 07:36:23

Re: Edexcel Decision Maths 1 (General Help)

#8 2012-04-15 07:50:49

Re: Edexcel Decision Maths 1 (General Help)

#9 2012-04-15 07:55:05

Re: Edexcel Decision Maths 1 (General Help)

#10 2012-04-15 08:01:24

Re: Edexcel Decision Maths 1 (General Help)

#11 2012-04-15 09:13:00

Re: Edexcel Decision Maths 1 (General Help)

#12 2012-04-15 10:10:34

Re: Edexcel Decision Maths 1 (General Help)

#13 2012-04-15 22:59:34

Re: Edexcel Decision Maths 1 (General Help)

#14 2012-04-15 23:20:49

Re: Edexcel Decision Maths 1 (General Help)

#15 2012-04-15 23:52:12

Re: Edexcel Decision Maths 1 (General Help)

#16 2012-04-16 00:00:04

Re: Edexcel Decision Maths 1 (General Help)

#17 2012-04-16 00:03:13

Re: Edexcel Decision Maths 1 (General Help)

#18 2012-04-16 03:13:10

Re: Edexcel Decision Maths 1 (General Help)

#19 2012-04-16 07:23:43

Re: Edexcel Decision Maths 1 (General Help)

#20 2012-04-16 08:27:42

Re: Edexcel Decision Maths 1 (General Help)

#21 2012-04-17 07:02:58

Re: Edexcel Decision Maths 1 (General Help)

#22 2012-04-17 07:45:54

Re: Edexcel Decision Maths 1 (General Help)

#23 2012-04-18 04:42:35

Re: Edexcel Decision Maths 1 (General Help)

#24 2012-04-18 05:31:02

Re: Edexcel Decision Maths 1 (General Help)

#25 2012-04-18 06:01:13

Re: Edexcel Decision Maths 1 (General Help)

Board footer