# Elevation (Hotseat: STEP I 2015)

#### Background

STEP (short for Sixth Term Examination Paper) is a somewhat difficult Maths exam that is used by several UK universities (including Cambridge, Warwick and Imperial) as part of a conditional offer for courses in Mathematics and Computer Science. A part of my conditional offer for Imperial included a 2 in any STEP paper of my choice, though it was paired with a relatively awkward 35 points for IB – perhaps because the rest of my portfolio was pretty strong.

There are three papers – I, II and III; III includes A Level Further Mathematics content, while I and II remain within the A Level Mathematics syllabus. I is also somewhat easier than II; that said, I think both papers exist because Cambridge does sometimes want students who didn’t take Further Mathematics to get a pair of grades in these exams. Nonetheless, STEP I itself is certainly no pushover. Students are graded on a scale of S, 1, 2, 3, U; the 2015 STEP I paper had 73.1 percent of students scoring at least ‘3’ (the lowest pass grade), and just 42.6 percent scoring at least ‘2’ (the lowest grade many universities would consider). This may be compared with A Level mathematics in 2015, where the analogous metrics of A*-E and A*-C respectively are 98.7 and 80.8 percent; and this is even before we factor in selection bias.

Each paper consists of 13 questions, but candidates are only required to pick six of them; their highest-scoring six questions will be used to determine their final score. Questions have equal weight (and each is marked with integers out of 20, which seems suspiciously similar to how I’ve seen this done at many universities!). Eight of the 13 questions are classified as “pure mathematics” and include tasks testing concepts like calculus, trigonometry, algebraic manipulation, series and even number theory. Three are classified as “mechanics”, typically requiring calculations on Newtonian mechanics, and two as “probability and statistics”. I almost always do 4/0/2 or 3/1/2. Note that it is possible to attempt seven or even more questions as a form of “insurance”, though given the strict time constraints this is likely to be difficult.

#### Performance

I had a fairly decent run, picking up 114 out of 120 points; mainly losing these to minor slips/cases where an important statement was not explicitly asserted, and a good chunk in question 7 with not clearly handling a case which was expected to be shown to bear no fruit (I thought it was rather obvious that it was not needed, and dismissed it in one line).

The last row indicates the order in which I attempted the problems; it seems this was generally consistent with how long I actually took on them (problems 2 and 13 were fast; 1 and 8 were somewhat in-between, and I struggled for a bit with 12 and messed up 7 while using up a fair bit of the time). Note that the “break-even” time if one wants to answer all questions would be 30 minutes per question.

#### Selected Problems in Depth

##### Problem 8: Series Division

First, prove that $1 + 2 + \ldots + n = \frac{n(n+1)}{2}$ and $(N-m)^k + m^k$ is divisible by $N$. Then, consider

$S = 1^k + 2^k + 3^k + \ldots + n^k$

Show that if n is a positive odd integer, then S is divisible by n, and if n is even then S is divisible by n/2. Show further, that S is divisible by $1 + 2 + 3 + \ldots + n$.

The two lead ins were simple. Induction does work but in both cases there were much better methods that could be used (write the series twice and pair terms up, and then pair terms in the binomial expansion). Later parts involved pairing S with a zero term, but the general theme of “pairing terms” persisted throughout the question. I think the toughest part of this one was, knowing that one had to show divisibility by $\frac{n(n+1)}{2}$ at the very end, figuring out that it was safe to split this into two terms and show them separately. This works because the highest common factor of $n$ and $n + 1$ is 1. My number theory was a bit rusty so I wasn’t sure if that was correct, and proved it during the paper.

##### Problem 12: On Fish Distributions

The number X of casualties arriving at a hospital each day follows a Poisson distribution with mean 8. Casualties require surgery with probability 1/4. The number of casualties arriving on each day are independent of the number arriving on any other day, as are the casualties’ requirements for surgery. (After some initial work) Prove that the number requiring surgery each day also follows a Poisson distribution and state its mean. Given that in a particular random chosen week 12 casualties require surgery on Monday and Tuesday, find the probability that 8 casualties require surgery on Monday (as a fraction, in its lowest terms).

This one really wasn’t too bad, though it involved a lot of algebraic manipulation and it seems I took quite a long time on it when doing the paper. Essentially, the independence condition should hint that if we have X casualties, the probability of S needing surgery is clearly binomially distributed. X itself is a random variable, but that’s fine; the law of conditional expectation gives us

$P(S = s) = \displaystyle \sum_{t = s}^{\infty} P(S = s | X = t) P (X = t)$

and a suitable substitution yields this:

$P(S = s) = \displaystyle \sum_{t = s}^{\infty} \left( \frac{t!}{s! (t - s)!} \times \left( \frac{1}{4} \right)^s \times \left( \frac{3}{4} \right)^{t-s} \times \frac{e^{-8} 8^t}{t!}\right)$

After some fairly involved algebraic manipulation, one can indeed recover a Poisson form for the pdf of S. Using this, the last part is relatively simple actually; we want $P(S_1 = 8 | S_1 + S_2 = 12)$ and relying on the fact that a sum of independent Poissons is Poisson itself (so the means of $S_1$ and $S_2$ are 2 each gives us that the mean of $S_1 + S_2$ is Poisson, and with mean 4).

##### Problem 13: Probability and State Machines

Already covered in a previous post. I dispatched this one very quickly, though I was already familiar with the Markov process model that I used here.

#### Meta-Analysis

The main datasource we have available here is an Examiner’s Report that discusses to some extent what happened (though we don’t have full details). The grade boundary for an S is 96, so a 114 is comfortably in that range; 3.5 percent of candidates scored that. Question-level data isn’t published beyond the comments in the Examiner’s Report.

Focusing on the questions that were attempted, my opener Q1 which was a test of curve-sketching was also the highest-scoring question on the paper, with students scoring an average mark of over 13.5 (with caveats that some students were worried about curve sketching and thus avoided it). This was the second “easiest” question as far as I was concerned.

The other pure maths questions I attempted (2, 7 and 8) were also popular, and attempted with varying degrees of success (questions 2 and 7 were the second and third highest scoring questions). Probability and Statistics for some reason seems to always cause issues for students attempting these papers, with mean scores in the 2-3 range, though having done Olympiads in the past (and specialising in combinatorics and probability) I understandably found these easier.

Generally, STEP I for me tends to be fairly tough but certainly comfortable, II is solidly difficult, and doing III often makes me feel like a dog behind a chemistry set (“I have no idea what I’m doing”), especially since I didn’t take Further Maths in high school.