Zooming Through Data

Lab 1D

Directions: Follow along with the slides and answer the questions in red font in your journal.

• Previously, we've looked at graphs of entire variables (By looking at all of their values).
  • Doing this is helpful to get a big picture idea of our data.
• In this lab, we'll learn how to zoom in on our data by learning how to subset.
  • We'll also learn a few ways to manipulate the plots we've been making to make them easier to use for analyses.
• Import the data from your class' Food Habits campaign and name it food.

• In lab 1B, we learned that we can facet (or split) our data based on a categorical variable.
• Use the dotPlot() function to create a dotPlot of the amount of sugar in our food data.
  • The code to create a dotPlot is exactly like you'd use to make a histogram.
  • Make sure to use a capital P in dotPlot.
• Split the dotPlot in two by faceting on our observations' salty/sweet variable.
  • Describe how R decides which observations go into the left or right plot.
  • What does each dot in the plot represent?

• It would be much easier to compare the sugar levels of salty and sweet snacks if the dotPlots were stacked on top of one another.
• We can change the layout of our separated plots by including the layout option in our dotPlot function.
  • Add the following option to the code you used create the dotPlot split by salty_sweet

layout = c(1,2)

• Hint: Use your history pane to see how we handled options with the bargraph function. Use a similar syntax to add the layout option to the dotPlot function.

• Subsetting is a term we use to describe the process of looking at only the data that conforms to some set of rules:
  • Geologists may subset earthquake data by looking at only large earthquakes.
  • Stock market traders may subset their trading data by looking only at the previous day's trades.
• There's many ways to subset data using RStudio, we'll focus on learning the most common methods.
  

• Creating two plots, one for salty and one for sweet is useful for comparing salty and sweet But what if we want to examine only one group by itself?
• Start by creating a subset of the data:
  • Fill in the blanks below with the data and variable names needed to filter our food data based on people who ate Salty snacks:

food_salty <- filter(____ , ____ == "Salty")

• View food_salty and write down the number of observations in it. Then use the subset data to make a dotPlot of the sodium in our Salty snacks.

• Coding in R is really just about supplying directions in a way that R understands.
  • We'll start by focusing on everything to the right of the “<-” symbol

food_salty <- filter(____ , ____ == "Salty")

• filter() tells R that we're going to look at only the values in our data that follow a rule.
• The first blank should be the data we're going to filter down into a smaller set (Based on our rule).
• salty_sweet == "Salty" is the rule to follow.

• We can decompose our rule, salty_sweet == "Salty", into 3 parts:
  • (1) salty_sweet, is the particular variable we want to use to select our subset.
  • (2) "Salty", is the value of the variable that we want to select. We only want to see data with the value Salty for the variable salty_sweet.
  • (3) == describes how we want to relate our variable (salty_sweet) to our value ("Salty"). In this case, we want values of salty_sweet that are exactly equal to "Salty".
• Notice: Values (that are also words) have quotation marks around them. Variables do not.

• We can use the head() function to help us see what's happening when we write salty_sweet == "Salty".
  • head() returns the values of the first 6 observations.
  • The tail() function returns the last 6 observations.
• Run the following code and answer the question below:

head(~salty_sweet == "Salty", data = food)

• What do the values TRUE and FALSE tell us about how our rule applies to the first six snacks in our data? Which of the first six observations were Salty?

• To use our subset data we need to save it first.
  • When we save something in R what we are really doing is giving a value, or set of values, a specific name for us to use later.
• The arrow <- is called the “assignment” operator. It assigns names (on the left) to values (on the right)
  • We now focus on everything to the left of, and including, the “<-” symbol

food_salty <- filter(____ , ____ == "Salty")

food_salty <- filter(____ , ____ == "Salty")

• This code then:
  • takes our subset data, (everything to the right of “<-”) …
  • and assigns the subset data, by using the arrow “<-” …
  • the name food_salty.
• We can now use food_salty to do anything we could do with the regular food data …
  • but only including those snacks who reported being Salty.

• We often want to filter our data based on multiple rules.
  • For instance, we might want to filter our food data based on the food being salty AND costing less than 2 dollars.
• We can include multiple filters to our subsets by separating each rule with a comma like so:

my_sub <- filter(food , salty_sweet == "Salty", cost <= 2)

• View the my_sub data we filtered in the above line of code and verify that it only includes salty snacks that cost less than 2 dollars.

• Use an appropriate dotPlot to answer each of the following questions:
  • About how much fat does the typical sweet snack have?
  • How does the typical amount of fat compare when healthy_level < 3 and when healthy_level > 3?
• It can sometime be helpful to change the number of intervals, or bins, used in dotPlots and histograms.
  • To change the number of intervals in your plots, include the nint option. For instance, to have a plot with 3 bins, use nint = 3. To have a plot with 30 bins, use nint = 30.