Lab 03 - Candy Regression

Published

September 20, 2022

Due: 2022-09-28 at midnight Turn your .html file in on Canvas

For this lab we are going to work with the Candy Power Ranking data. This is the data behind the FiveThirtyEight story “The Ultimate Halloween Candy Power Ranking”.

Getting started

Go to RStudio Pro and click:

Step 1. File > New Project
Step 2. “Version Control”
Step 3. Git
Step 4. Copy the following into the “Repository URL”:

https://github.com/sta-112-f22/lab-03-candy-regression.git

Packages

In this lab we will work with the tidyverse and fivethirtyeight packages. We start by loading the packages.

library(tidyverse)
library(fivethirtyeight)

Note: Make sure you render your HTML document frequently - this will also save your .qmd file! This is good practice to make sure you are catching any bugs early (and saving your output regularly).

Note that these packages are also loaded in your document.

Load the data

The data we will be working with today is called candy_rankings and it’s in the fivethirtyeight package.

To find out more about the dataset, type the following in your Console: ?candy_rankings A question mark before the name of an object will always bring up its help file. This command must be ran in the Console.

candy_rankings is a tidy data frame, with each row representing an observation and each column representing a variable.

To view the data, run data(candy_rankings) and click on the name of the data frame in the Environment tab.

You can also take a quick peek at your data frame and view its dimensions with the glimpse function.

glimpse(candy_rankings)

The description of the variables, i.e. the codebook, is given below.

Header	Description
`chocolate`	Does it contain chocolate?
`fruity`	Is it fruit flavored?
`caramel`	Is there caramel in the candy?
`peanutalmondy`	Does it contain peanuts, peanut butter or almonds?
`nougat`	Does it contain nougat?
`crispedricewafer`	Does it contain crisped rice, wafers, or a cookie component?
`hard`	Is it a hard candy?
`bar`	Is it a candy bar?
`pluribus`	Is it one of many candies in a bag or box?
`sugarpercent`	The percentile of sugar it falls under within the data set.
`pricepercent`	The unit price percentile compared to the rest of the set.
`winpercent`	The overall win percentage according to 269,000 matchups.

Exercises

How many observations are in this dataset? What does each observation represent?
How many variables are in this dataset? Describe the variable types - are they numeric / continuous / quantitative or are they categorical? If they are categorical, what are the categories? Are they a special type of categorical variable known as a binary variable, a categorical variable with just two levels?
Create a new dataset called single_candy_rankings that only contains observations where pluribus == 0. This new dataset contains only candies that come with one candy (instead of many in a bag or box). The rest of this lab will use this new dataset.

single_candy_rankings  <- candy_rankings %>%
  filter(---)

Let’s take a closer look at two variables, sugarpercent and winpercent. Summarize the center and spread of these two variables by calculating the mean and standard deviation using the code below and describe what you see. Create two histograms using the single_candy_rankings dataset, one of sugarpercent and one of winpercent. Be sure to select an appropriate binwidth.

single_candy_rankings %>%
  summarise(mean_sugarpct = mean(sugarpercent),
            mean_winpct = mean(winpercent),
            sd_sugarpct = sd(sugarpercent),
            sd_winpct = sd(winpercent))

Using the single_candy_rankings, create a visualization to describe the relationship between sugarpercent and winpercent (where winpercent is on the y-axis). Describe the relationship that you see.
Examine the correlation between sugarpercent and winpercent using the cor() function by modifying the code below. Comment on the correlation coefficient - how does it compare to what you saw in Exercise 5.

single_candy_rankings %>%
  summarise(r = cor(---))

In the special case of a simple linear regression, you can calculate \(\hat{\beta}_1\) without having to use any calculus! It turns out the following equation works:

\[\hat{\beta}_1 = \hat{r} \frac{\hat{\sigma}_y}{\hat{\sigma}_x}\]

Where \(\hat{r}\) is the estimated correlation between \(x\) and \(y\), \(\hat{\sigma}_x\) is the standard deviation of \(x\), and \(\hat{\sigma}_y\) is the standard deviation of \(y\). Let’s see if we can calculate this. Edit the code below to calculate:

sd_x: the standard deviation of sugarpercent
sd_y: the standard deviation of winpercent
r: the correlation between sugarpercent and winpercent
beta1: The slope of the simple linear regression between winpercent ~ sugarpercent (I have written the equation for beta for you, you do not have to edit this final part).

single_candy_rankings %>%
  summarise(sd_x = ---,
            sd_y = ---,
            r = ---,
            beta1 = r * (sd_y / sd_x))

We are interested in fitting a simple linear regression of the form:

\(y = \beta_0 + \beta_1x + \epsilon\)

where \(y\) is winpercent and \(x\) is sugarpercent. Use the lm() function to fit this model.

What is \(\hat{\beta}_0\)? What is \(\hat{\beta}_1\)? Interpret these results in words. How does this \(\hat{\beta}_1\) compare to the one you calculated in Exercise 7?
Calculate the residuals for the model fit in Exercise 8. Create a scatterplot of the residuals by \(\hat{y}\), the predicted outcome, winpercent. Do the conditions of linearity and constant variance seem to hold?
Plot the residuals using a histogram with binwidth = 9 to observe whether they follow a bell-shaped distribution. Comment on what you see.
Calculate a confidence interval for \(\hat\beta_1\). Report this value and interpret it in the context of this model.
We are interested in testing whether there is a relationship between winpercent and sugarpercent. The null hypothesis is that \(\beta_1=0\) What is the alternative hypothesis? How can you conduct this hypothesis test?
Report the p-value for the hypothesis test for Exercise 13. What does this mean?