Theme of the course: what do you do to get answers from data? Previous steps: get the data; clean it up until you can work with it.

One possible next step: use statistics to make inferences about what it means.

My goals for this part of the course are fairly modest: you should be able actually *do something* with statistics.

I'll be happy with probably enough

.

Corollary: if doing statistical analysis wrong would have serious consequences (someone's health, loss of a bunch of money, etc), then ask somebody who knows more statistics than this course will cover.

Let's review some of those things you Definitely Know™ from your prerequisite statistics course…

- Quantitative
- Numeric values that have magnitude: -4.2 vs 18.9.
- Ordinal
- Ordered values or categories with no magnitude: unsatisfied/neutral/satisfied, 0–9/10–19/20–29/30–39, A+/A/A-/B+/….
- Nominal
- Unordered properties or categories: Vancouver/Ottawa, red/green/blue, control/treatment.

We generally think of quantitative data as data

, but the different categories come up.

We're usually concerned about the population: all of the values. We want to come to conclusions about the entire population. e.g.

Are men taller than women?

≈Is the average height of all men larger than the average height of all women?

Should we put item X on sale?

≈Will we make more profit (from all of our customers' purchasing decisions) if the cost of X was lower than its current value?

But we don't usually get to look at the entire population (especially if it's extremely large or infinite). We usually have to deal with just a sample: a subset of the population. e.g.

- 50 men and 50 women: measure their heights.
- A fraction of customers who are offered the lower price, compared to the rest.

The point of inferential statistics is to use (well-chosen) samples to come to (probably-correct) conclusions about the population.

- Yes, the average height of men is larger than the average height of women.
- No, don't put X on sale: we think it will make less money.

If we have some random thing happening (a random variable, like sampling an individual from a population), what is the probability of a certain outcome? (e.g. height = 1.80 m, heads/tails).

A probability distribution is the description of probabilities for all outcomes.

A discrete probability distribution has outcomes from a discrete (usually finite) set. e.g. flipping a coin, number of times a Wikipedia page will be viewed tomorrow.

Of course, the sum of probability of every possible outcome must be one.

\[ \sum_{u\in U} P(u) = 1 \]For a discrete or continuous probability distribution, we can talk about the cumulative distribution function, the probability of the outcome being less-than-or-equal-to a particular value. Often written \(F(x)\).

Or its derivative, the probability density function, often \(f(x)\).

The picture is probably more useful. A normal distribution's cumulative distribution function, and probability density function:

Where is the center

of your data?

Most commonly used: the mean or expected value.

For the population, these are the same, and usually called \(\mu\) or \(E(X)\).

A sample has a mean (but no expected value), \(\overline{x}\).

The sample mean \(\overline{x}\) is an unbiased estimator of the population mean \(\mu\): if the sample is randomly chosen, \(E(\overline{x})=\mu\).

i.e. If you take a good sample and calculate the mean, you have a meaningful estimate the population mean.

Can also look at the median: value in the middle when sorted; 50th percentile.

Or the mode: value that occurs most frequently.

How spread out is the data? How far away from the mean are the points usually

found?

Most commonly used: standard deviation of a population \(\sigma\), or a sample \(s\). Or variance: \(\sigma^2\) or \(s^2\).

Again, the sample standard deviation is an unbiased estimator of the population standard deviation: \(E(s)=\sigma\).

In general, at least half of a population is within \(\sigma\sqrt{2}\) of the mean. For a normal distribution, ≈68% is within \(\sigma\).

So *mean* is something like where is the middle of the data?

The *standard deviation* is how spread out is the data from the mean?

Pandas can make quick work of showing you summary stats for a DataFrame:

print(data.describe())

id rating timestamp count 1.669000e+03 1669.000000 1.669000e+03 mean 8.245327e+17 11.762133 1.485419e+09 std 9.829935e+16 1.646146 2.343639e+07 min 6.989080e+17 0.000000 1.455468e+09 25% 7.486928e+17 11.000000 1.467337e+09 50% 8.026004e+17 12.000000 1.480190e+09 75% 8.834828e+17 13.000000 1.499474e+09 max 1.125920e+18 17.000000 1.557275e+09

Pandas doesn't know that id

is actually nominal, so that column isn't meaningful.

With two or more variables, how are they related?

The covariance (\(\mathit{cov}(X,Y)\) or \(\sigma_{X,Y}\) for populations, \(s_{X,Y}\) for samples) gives information about the joint variability: do they change together or independently?

Note: \(s_X\) is sample standard deviation and \(s_X^2\) is variance, but \(s_{X,Y}\) is sample co**variance**

Positive covariance: larger \(Y\) usually happen with larger \(X\). Negative covariance: larger \(Y\) usually happen with smaller \(X\).

The correlation coefficient is basically the same info, but normalized into a -1 to 1 range. \(\rho\) (rho) for populations, \(r\) for samples.

\[\begin{align*} \rho_{X,Y} &= \tfrac{\sigma_{X,Y}}{\sigma_{X}\sigma_{Y}} \\ r_{X,Y} &= \tfrac{s_{X,Y}}{s_{X}s_{Y}} \end{align*}\]Values close to -1 and 1: a lot of **linear** relationship between the variables. Close to 0: little or no linear relationship.

If you're interested in two variables' correlation coefficient, we also have a quick tool for that:

print(stats.linregress(data['timestamp'], data['rating']).rvalue)

0.5005674118565123

Remember that \(r\approx 0\) for some data means there's no apparent *linear* relation, not that \(x\) and \(y\) aren't related to each other.

This data has \(r=0.003\), but has a fairly obvious relationship between \(x\) and \(y\).

The basic summary stats can only tell you so much: they are often not enough information to really understand what's happening in a data set.

You should start by plotting your data, even if you don't need

a plot. It can tell much more of a story.

Anscombe's quartet: four data sets each with \(\overline{x} = 9.00 \), \(\overline{y} = 7.50 \), \(s_x^2 = 11.00 \), \(s_y^2 = 4.125\), \(r_{xy} = 0.816\), regression line \(y=3.00 + 0.500x\). *

The lesson: start by making a quick plot of your data.

It can tell you a lot that basic summary stats can't. (But summary stats can often tell you things a plot can't.)

[Also, outliers can really affect your analysis.]

For two-dimensional data, a scatter plot is often the most obvious: shows you the approximate distribution of both variables, and any obvious relationships.

For one variable, maybe a box plot. Shows the median, quartiles, range, outliers.

plt.boxplot(data['rating'], notch=True, vert=False)

Or a histogram: a look at the rough shape of the distribution.

plt.hist(data['rating'])

There are several probability distributions that are often interesting.

Some of them are used to come to a conclusion while inferring something about the data: more later.

For example, we may come up with a random process and realize if

`H` is true and we do [a bunch of arithmetic], then we get a value sampled from a Student's T

distribution.

What if we do that and find a value of 3.17? There's a <10% probability of sampling a value that far from the mean, so it seems unlikely that `H` is true.

… maybe that's useful to notice.

One in particular that comes up a lot: the normal distribution, generally written \(\mathcal{N}(\mu, \sigma^2)\).

e.g. for Kalman filters, we assumed the noise was normally-distributed (with \(\mu=0\) and we guessed \(\sigma^2\) as closely as we could).

It's very common to get normally-distributed values when doing random sampling.

e.g. flip \(n\) coins: number of heads is distributed \(\mathcal{N}(\frac{n}{2},\frac{n}{4})\) (if \(n\) is large).

The central limit theorem (more later) says that you can get a normal distribution anywhere if \(n\) is large enough, and you look at your data the right way.