Don't do MA305 - given that you've done no abstract maths, it's not a good choice I think. I mean I know you know what a function space is, but it's one of the tougher maths courses and I'd expect it to be quite abstract in places. For example, you probably have no idea what uniform convergence means, but I'm pretty sure it'll be a big part of MA305. You CAN probably do it with little to no knowledge of abstract maths (after all, they will teach you what uniform convergence is), but the point is that your mind hasn't been trained in the right way to do a course like MA305, even though you don't necessarily need the KNOWLEDGE of abstract maths (you just need the right thought process). That's just my opinion though, but I haven't actually studied MA305, so you should e-mail Dr. Sasane and ask.
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MA301 is supposed to be really easy, the second half (covered in MA300) is not easy, but doable. I don't think it's gonna be much like EC202, I think it's more like proving stuff, but it's supposed to be fairly straightforward.
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FM212 is a waste of time in my opinion if you've done or are going to do a good internship over the summer or something. It probably is the easiest option out of your choices in the sense that it's not intellectually tough, just a lot of boring stuff to learn (it's definitely not easy to get a First in this course though, probably still EASIER than your others).
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Labour is not supposed to be hard - it'll be easier than EC301 and EC307 for sure. It's kind of just average. I think the problem people have with it is that the lecturer doesn't use slides, s/he writes on the whiteboard like school and you sit there copying it down I've heard.
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EC309 has a term of probability theory. What this means is that you'll learn how to think about probabilities FORMALLY, you'll talk about exactly what a probability space is, how you define a sigma field on that space (which is a set of events) and then a probability measure on that space. The aim is to basically get to the point of being able to talk about the law of large numbers properly. You know how in EC221 they just say "by some law of large numbers, if the stuff is i.i.d then the sample moment converges in plim to the population moment"? You'll talk about what plim really means and why this convergence works. You'll learn there are different modes of convergence:
Pointwise convergence ==> almost sure convergence ===> plim convergence ==> distributional convergence
Converge in mean ==> plim convergence ==> distributional convergence.
You'll learn what all those convergences mean, when/why one implies the other, and you'll learn how to apply it to various settings. You'll learn stuff like what exactly is a distribution function (CDF) - you learn the 4/5 conditions in ST102, now you need to be able to prove them (stuff like as x tends to infinity, CDF tends to 1; CDF must be right continuous; etc). Then you finally learn about the different types of law of large numbers and how they work.
It's only AFTER you go through this hell that you start doing econometrics in the second term, where it's stuff like 3 staged least squares.
If you're at all interested in doing a Masters, this is a good course. If the first term stuff seems interesting, this is a good course. Otherwise, go with EC333 where I assume it's more like "hmm, so Gauss Markov assumptions don't work in this case, what can we do?"
(bear in mind I've not studied either courses, but I have friends that have done EC309 and I've studied Probability for Finance).
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You can see a rant by me about EC301 in some posts I made a few days ago (just search through my posts).
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As for Development Economics, this is a really tough module. You're taught through actual journal articles that are really meant to be for academics, so it's a lot of reading and it's tough. There is a textbook but I doubt it covers the material properly. Everyone I know who takes this course really hated it around Easter, but at the same time it's a really rewarding course I think if you can put up with all the reading and writing. There's an optional project involved too where you can do some research work if you're interested.
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As a general tip, if you're not on 4 or more Firsts and you're looking to get a First, do yourself a favour and pick one very easy course on purpose. Even if you have little interest in it/it doesn't fit in with your plans for the future, just pick it. It's one of my biggest regrets right now. EC301, EC307 and EC309 are definitely very, very tough courses.
Hope this helps in some way.