Homework 1

Due: February 23, 2018 @ 11:59pm

Instructions

Obtain the Github repository you will use to complete homework 1, which contains a starter RMarkdown file named homework_1.Rmd, which you will use to do your work and write-up when completing the questions below. Be sure to save, commit, and push (upload) frequently to Github so that you have incremental snapshots of your work. When you’re done, follow the How to submit section below to setup a Pull Request, which will be used for feedback.

  • Remember that the point of us using RMarkdown documents is to combine code and writeups! Each block of R code should have some sort of explanation or justification using full sentences.

  • Your grade will take into account your code, your explanations, and whether your document looks nice when “knitted” to HTML or PDF.

The rail trail dataset

plot of chunk mass-central-logo

For this homework assignment, you will be working though a set of visualization problems based on the rail_trail dataset. The rail_trail dataset was collected by the Pioneer Valley Planning Commission (PVPC) and counts the number of people that walked through a sensor on a rail trail during a ninety day period. A rail trail is a retired or abandoned railway that was converted into a walking trail. The data was collected from April 5, 2005 to November 15, 2005 using a laser sensor placed at a location north of Chestnut Street in Florence, MA.

plot of chunk florence-ma-chestnut

The dataset contains the following variables:

Variable Description
hightemp daily high temperature (in degrees Fahrenheit)
lowtemp daily low temperature (in degrees Fahrenheit)
avgtemp average of daily low and daily high temperature (in degrees Fahrenheit)
season indicates whether the season was Spring, Summer, or Fall
cloudcover measure of cloud cover (in oktas)
precip measure of precipitation (in inches)
volume estimated number of trail users that day (number of breaks recorded)
weekday indicator of whether the day was a non-holiday weekday

Questions

When describing the contents of a visualization, follow the general ideas summarized here: http://spring18.cds101.com/materials/describing-univariate-and-bivariate-data/

  1. In the rail_trail dataset, how many rows are there? How many columns? Which variables in the dataset are continuous/numerical and which are categorical?

  2. Create a histogram of the variable volume using the following code:

    ggplot(data = rail_trail) +
  geom_histogram(mapping = aes(x = volume))

    Describe the shape and center of the distribution. Afterward, try adjusting the size of the histogram bins by adding the binwidth input. To start with, use binwidth = 21. If you need help with where to place binwidth, read the documentation by running ?geom_histogram in your Console window. Then, find a binwidth that’s too narrow and another one that’s too wide to produce a meaningful histogram.

  3. Create a histogram for each of the remaining numerical variables, and describe the shape and center of each distribution. Are there any distributions that are similar in shape to each other?

  4. Use geom_point() to create a scatterplot that plots weekday versus season. Why is this plot not useful?

  5. Create a geom_count() plot (an alternative to a mosaic plot) using the same variables you considered in question 4:

    ggplot(data = rail_trail) +
  geom_count(mapping = aes(x = season, y = weekday))

    Which circle in the plot takes up the most area? Explain the meaning of the different size circles in the plot and what information it contains that is missing in the previous scatter plot.

  6. Run ?geom_bar in the Console window and read the documentation for geom_bar(), and then look at the entry for it on the ggplot2 cheatsheet Use geom_bar() to reproduce the following bar chart:

    plot of chunk seasons-bar-chart

    After reproducing the plot, explain what the height of each bar means.

  7. Starting from the code snippet you deduced in question 6, create two more bar charts:

    • Create a bar chart by supplying the input position = "dodge" to geom_bar()

    • Create a bar chart by supplying the input position = "fill" to geom_bar().

    After creating the visualizations, describe the feature that position controls.

  8. Create a bar chart that maps its aesthetic aes() to precip > 0. Interpret what this bar chart means.

  9. Create a scatter plot of volume versus hightemp using geom_point(). Describe any trends that you see.

  10. Take the code snippet you wrote for question 9 and map the weekday variable to color. Then create a second plot where, instead of mapping weekday to color, you facet over weekday using either facet_wrap() or facet_grid(). Discuss the advantages and disadvantages to faceting instead of mapping to the color aesthetic. How might the balance change if you had a larger dataset?

  11. Take the code snippet that you wrote down in question 10 that faceted over weekday and create a model for each facet panel using geom_smooth(). Discuss the trends in the number of rail trail users that geom_smooth() picks up.

  12. Copy the code snippet you deduced in question 11 and use the input se = FALSE for geom_smooth(). What does the se input option for geom_smooth() control?

How to submit

When you are ready to submit, be sure to save, commit, and push your final result so that everything is synchronized to Github. Then, navigate to your copy of the Github repository you used for this assignment. You should see your repository, along with the updated files that you just synchronized to Github. Confirm that your files are up-to-date, and then do the following steps:

  1. Click the Pull Requests tab near the top of the page.
  2. Click the green button that says “New pull request”.
  3. Click the dropdown menu button labeled “base:”, and select the option starting.
  4. Confirm that the dropdown menu button labled “compare:” is set to master.
  5. Click the green button that says “Create pull request”.
  6. Give the pull request the following title: Submission: Homework 1, FirstName LastName, replacing FirstName and LastName with your actual first and last name.
  7. In the messagebox, write My homework 1 submission is ready for grading @shuaibm @jkglasbrenner.
  8. Click “Create pull request” to lock in your submission.

Cheatsheets

You are encouraged to review and keep the following cheatsheets handy while working on this assignment: