Statistics and Visualisation with R #3

Post number three of my notes from a course on Statistics and (data) Visualisation with R, presented by Lucia Michielin of the University of Edinburgh in June and July.

Third week

• Programme overview
• Class 5 - Data Collection Bias, Probability, and Distribution
  • Data Collection and bias
  • Probability
  • Distribution
• Class 6 - Hypothesis testing and the main tests
• Playtime discoveries
• Next steps: other ways to learn R
• Notes and references

Programme overview

Class 5 - Data Collection Bias, Probability, and Distribution

After a quick review of the challenge from last week, an agenda for today was shared which wasn’t really followed¹.

Data Collection and bias

“Data collection is the process of gathering and measuring information on targeted variables in an established system, which then enables one to answer relevant questions and evaluate outcomes. The goal for all data collection is to capture quality evidence that allows analysis to lead to the formulation of convincing and credible answers to the questions that have been posed.” (Wikipedia)

The sample is taken from the population, but a bias is introduced when the sample is taken. One example of this is the survivorship bias.

Probability

A basic definition was given to the class, with an example from an earlier class that helped us. If we make the statement if **A** is true then **B** is true can we infer from this that if **B** is true then **A** is true? Clearly not, but we might be able to say if **B** is true then **A** is more plausible. Plausibility here relates to how probably something can be.

“Probability is a numerical description of how likely an event is to occur or how likely it is that a proposition is true.” - Wikipedia

Distribution

In introducing this topic, we were shown the really neat rnorm() function that generates random data sets with specified mean and standard deviation.

In the class, we were given a challenge to go through the steps needed with a new data set to examine it. These steps seem to be just to plot it and have a look. All of the curves looked to be distributed around a central mean, which makes them normal distributions. There was a problem with the challenge files which meant that some of us didn’t have the correct files and so did a different (trivial) task. The key message is to examine your data as soon as you have it, to look for central values and distribution, so as to understand it and how you expect it to behave in analysis.

Other distributions include uniform, logarithmic, left-, right-skewed and bimodal.

Class 6 - Hypothesis testing and the main tests

Just half a dozen students today. We started with a review of the mini-challenge from last session, which included a consideration of colours: HTML colour and a ggplot colour reference were shared.

One emphasis today is that there are multiple ways of doing the same thing in R, with an illustration of data importation and summarisation. Although some of the few remaining students seemed to be following the narrative, I became increasingly frustrated and lost trying to follow the class: the thread was along the lines of hypothesis testing, the null hypothesis and corrections.

Playtime discoveries

Between this week’s two classes, I had a quick look at the Comprehensive R Archive Network (CRAN) and discovered a neat thing. Remember the funky assignment operator <- from the first class which had its funky keyboard shortcut alt -? Most programming languages I’ve used have a simple equals sign as the assignment operator and I was a bit baffled as to why super-sexy R should do something so weird as <-. Well, it turns out that in most contexts, = works in exactly the way you’d expect, so these are equivalent:

> x <- 99
> x = 99

What’s interesting about the R syntax, though, it that it can be reversed and it still works:

> 99 -> x

I’m sure that the case for this will become apparent eventually. One more way to do assignments is to call the assignment function:

> assign('x', 99) # notice the weird need for quotes around x in the function call

Next steps: other ways to learn R

I got totally overwhelmed with the rate of new concepts in this class, or perhaps the pace and style of delivery. I think, given the time I have to spend after each class going over the material in order to make sense of it, it is clear that this course is not right for me. I have decided to abandon it and follow other materials more appropriate to my purpose, which is to develop skills in data visualisation using R. Despite the title, this course is not doing that for me.

Having decided² to seek out other ways to learn how to R, I found that there are lots of self-directed learning resources out there, of course. An introduction to R course at DataCamp will be the new direction of travel down this rabbit hole for me.

Notes and references