April '07

[Benny]

So.

How did we do?

I quite liked it, actually...

Chromosomes questions took some thinking, and can't do likelihoods ever, but... yeah. Best CT4 paper yet, by some margin!

 

[Louisa]

That's not what I wanted to hear Benny.

Found it pretty evil - I wrote loads and still didn't finish various bits. And too many loose ends where they didn't specify a method or what data was available. Thinking about it after, I know I was some way out on several of those.

Oh and i set up states for the airport one the wrong way round, which made that qn messy.

Think it was helping me in the past papers to have the Markov bits and the life bits neatly separated. It all seems a bit more chaotic now...

 

[Nadeem_Ladha]

doh...and double doh

 

[Monkey_Mike]

I thought it was reasonably OK

the airport one was a bit chaotic...

so was the 2 step transition matrix, took ages

 

[Brickie_Burns]

I thought it was a pile of pants.

Took forever to do the two step transition matrix question. Didn't finish it in the end, needed to solve a quadratic right?.................forgot the formula.

Couldn't get my head around the airport question.

Couldn't get my head around the chromosome question.

I think I understand CT4 concepts well, but couldn't understand exactly the situations they wanted them applied to.

 

[decrement]

i thought the 2 step matrix q was easy. but no quadratic to solve, once u sub everything in it all falls out simply

chromosome one was good too, but i didnt get an irreducible chain, so there were 2 stationary ditributions

 

[Monkey_Mike]

ummmm decrement, a few of us got a quadratic needing to be solved to get the final 2 values...

it factorised pretty easy too

the airport one was a biatch... pardon my french

 

[Louisa]

I also got the quadratic on the 2-step matrix question - sorry decrement! That was actually one of the (few) questions i liked, thought it came out quite quickly for the number of marks involved.

The chromosome question took me far too long to understand what the states actually were - long time since i did any biology! I thought there's only one stationary distn, with half M non-carriers and half F non-carriers. If there's any carriers in it they fizzle away over time, as all the partners are non-carriers. But that doesn't seem enough work for 4 marks.

Think the airport one might have been okay if I'd done it near the beginning, did a lot of that sort of thing in a uni course on Markov chains. As it was I set it up backwards and didn't explain anything and they probably can't read what I wrote anyway.

 

[decrement]

hmm not sure where i went wrong with that last step then on the p^2 question

for the chromosome question there were 4 states and it wasnt irreducible right. because from XX (say) u cant get to X*X?

 

[Monkey_Mike]

Louisa,

you're right about the genetic disorder fizzling out over time...

If you did it with the aP = a equations, they came out as a1 = 0, a4 = 0

which left a2 = a3 ... 50% OK Male, 50% OK Female

 

[Leila]

Can the people who used a quadratic on the p^2 question remember any of the numbers in their matrix? I nearly got to a quadratic, but then I think I used another sum of probabilities =1 and avoided it. It would have been simple to check whether my answer was correct using the information in the question, but I didn't have time!!
Generally I thought it was a very strange paper - not very CT4 at all!

 

[Louisa]

So all we have to do to eradicate the disease is to ban carriers from
a) having kids together or
b) having more than one child with anybody else
:-/

decrement - yes, I agree it was not irreducible, for the reasons you give. That doesn't mean there has to be more than one stationary distn, just that there could be.

 

[Ryback]

The exam was way harder than any past papers imo. Almost every question was non-standard which meant more thinking time. Ran out of time really for airport question to be done properly, reckon i could have done it with about 45 mins worth of time, was a toughy and an unexpected type of question.

Rest wasn't too bad i guess, did the chromosome one slightly different, grouped male non carriers and female non carriers together because statistically didnt matter, again i made the s.d. = (0,0,1) there were no others despite chain being reducible becuase there was an absorbing state.

2 state MC one was something like:
(0.75, 0.25, 0)
(0.3, 0.2, 0.5)
(0, 0.5, 0.5)
but can't remember the correct ordering in each row.

Have a bit of a mental block on the rest of the paper, can't remember many of the other questions hope i got enough, i reckon they might scale it coz of tough paper though

 

[Deniese]

2 state MC one was something like:
(0.75, 0.25, 0)
(0.3, 0.2, 0.5)
(0, 0.5, 0.5)
but can't remember the correct ordering in each row.

looking at the transition diagram ..i dont believe state A went into B.

 

[Ryback]

Was probably (0.75, 0, 0.25) then, as i said i couldnt remember each row ordering exactly. How do you know have you got a copy of the paper or just from memory? Would like to see that last question again

 

[psychopath 0_o]

Was probably (0.75, 0, 0.25) then, as i said i couldnt remember each row ordering exactly. How do you know have you got a copy of the paper or just from memory? Would like to see that last question again

Yup you are right, well at least that's what I have got. since A cannot go to B but to C.

Heyhoo, I have got myself 8marks!!!

But I really have to say the paper is an OK paper, but strange.
I am not too sure how I did it. But I can say that this paper seems like asking you to apply what you know about CT4 to situations, this is the strange bit about this paper. Not like the past papers just give you some easy scenario and work on it.

I really want to get over this horrible subject!!!! Just a torture.
But strange......