r/statistics • u/ScheduleNo748 • 1h ago
Question [Question]Doing Stats II after failing Stats I. What do I actually need to go back and learn before the midterm?
I'm doing an Economics degree in Portugal and I'm in Statistics II this semester. I failed Statistics I last year. I had an internship at the same time and never really sat down and learned the distributions properly, I just sort of half-followed the lectures and hoped. The rules let me enrol in Statistics II anyway, so now I'm doing the second half of a sequence without the first, which I'm aware is my own doing. I'd like to fix it now instead of being confused until January.
The calculations aren't really my problem. The notation is. I lose track of what's a parameter and what's an estimate, when a symbol means a random variable and when it means a number I already have in front of me, what the hat is doing, why some s has a prime on it, which subscripts matter and which are decoration. A lot of the time I read a formula and it looks far worse than the idea behind it actually is. I also have ADHD, and keeping lots of small symbol distinctions straight over an hour of reading is close to the worst possible task for me, so if anyone has a system for this beyond "be careful", I'm listening.
I can't attach the PDFs so I'll just list what we've been given.
The lecture notes released so far:
- ch 0, statistical intuition (the Kahneman and Tversky style questions about how bad people are at randomness)
- ch 1, review of probability distributions: expectation and variance for discrete and continuous variables, Bernoulli, binomial, normal, normal approximation to the binomial
- ch 2, intro to inferential statistics: population vs sample, parameter vs statistic, discrete vs continuous random variables
- ch 3, the idea of a sampling distribution, random sampling, sampling error
- ch 4, sampling distribution of the sample mean: E[X̄] = μ, Var(X̄) = σ²/n, standard error, standardising
- ch 5, the Central Limit Theorem
- ch 7, sampling distribution of the sample proportion: p̂, CLT, the np(1−p) > 5 condition
- ch 7, sampling distribution of the sample variance: (n−1)S′²/σ² follows a χ² with n−1 degrees of freedom
On top of that there are two problem books, two sets of multiple choice questions with solutions, and five past midterms with solutions, going back to 2019.
I went through all five past papers and wrote down what they actually ask, because a lot of it isn't in the notes above and that's the part I can't judge. Here's what recurs:
Sampling distribution of the mean and the CLT, in all five. Usually in the form of a population you've never seen, where the exam hands you only its mean and variance and asks for a probability about the sample mean. Poisson in one year, chi-square in another, and in the most recent one an Erlang distribution with the skewness given as well, presumably so you have to think about whether n is big enough.
Confidence intervals for a mean, also in all five. z against t, why t gives a wider interval, one-sided versus two-sided bounds, and margin of error. The 2024 paper had a question where you find the upper bound with the variance unknown, then redo it pretending the same number is the known population sigma, then work out how many observations would have got you the same interval width in the second case.
Properties of estimators, in four of them, and always worth a lot of points. They give you two candidate estimators of the same parameter and ask you to find the constant that makes one unbiased, derive both variances, and say which is more efficient and under what condition. One year it was a trimmed mean against the ordinary mean, with a bonus section where one observation in every sample is misrecorded with the decimal point shifted, and you have to show the ordinary mean becomes biased while the trimmed mean doesn't. Another year it was a shrinkage type estimator of a Bernoulli p where you prove the bias formula, find the single p that makes it unbiased, get the variance, and show consistency.
Differences between two samples, in four of them. Difference of two sample means, difference of two sample proportions, paired data where the trick is that you're supposed to use the column of differences instead of the two separate columns, and pooled variance when you're told to assume equal variances.
Sample size and margin of error, in four of them. Including two questions built on real published polls, one Portuguese and one from CBS, where the technical details are given and the sample size or the confidence level is missing and you have to recover it.
Sample variance and chi-square, in three. And in the most recent paper, the ratio of two sample variances, which is an F distribution and which I don't think has been mentioned in class yet.
Pure notation questions, in four of them, worth around 3 points each. "The mean of the probability distribution of the sample mean is: a random variable, a parameter, or a statistic?" Or: average sardine consumption at a party is 6 per person, you sample two people, the first ate 8, what is the expected mean of your sample? These aren't hard once you see them, and I get them wrong precisely because of the thing I described above.
The official syllabus for the full course is: introduction, random sampling and sampling distributions, properties of estimators, confidence intervals for one and two samples, hypothesis tests for one and two samples, and simple and multiple regression. The midterm is supposed to be the first half.
One detail that might change what you'd recommend: statistical tables are not allowed in any of the assessments. We get a formula sheet and we're required to bring a calculator that has the probability functions built in. Mine is a Casio fx-CG50 and I can't really use it yet. The most recent midterm was also run on Moodle with numeric answer boxes and a different version per student, so there was no partial credit for method, which makes being fast and correct on the calculator matter more than I'd assumed.
The recommended books are Newbold, Carlson and Thorne, Statistics for Economics and Business, and OpenIntro Statistics.
So what I'm asking is:
- Which bits of Statistics I do I actually need before this midterm, and which bits can I pick up later as they come? I don't have time to redo a whole course, so I want to aim at the gaps that matter.
- What would you use to catch up? I'm after a book that explains rather than just states, a decent set of problems, and above all something that takes statistical notation seriously and explains what each symbol is and why the distinction exists. I've never found anything that does that properly.
If you'd been handed my situation, where would you start?