Releveling factors in R Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to change the reference level of the factor to "B".

R Programming

f <- factor(c("A", "B", "C"))
f <- relevel(f, ref = [1])
levels(f)

Drag options to blanks, or click blank then click option'

A"B"

B"C"

C"A"

D"D"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a level name that does not exist in the factor.

Forgetting to put the level name inside quotes.

The relevel() function changes the reference level of a factor. Here, setting ref = "B" makes "B" the first level.

2fill in blank

medium

Complete the code to set the reference level of the factor colors to "green".

R Programming

colors <- factor(c("red", "green", "blue"))
colors <- relevel(colors, ref = [1])
levels(colors)

Drag options to blanks, or click blank then click option'

A"red"

B"green"

C"blue"

D"yellow"

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a level not present in the factor.

Not assigning the result back to the variable.

Setting ref = "green" makes "green" the reference level in the factor colors.

3fill in blank

hard

Fix the error in the code to correctly relevel the factor f to have "low" as the reference level.

R Programming

f <- factor(c("high", "medium", "low"))
f <- relevel(f, ref = [1])
levels(f)

Drag options to blanks, or click blank then click option'

A"low"

Blow

C'low'

Dlow"

Attempts:

3 left

💡 Hint

Common Mistakes

Not using quotes around the level name.

Using mismatched or incomplete quotes.

The reference level must be a string enclosed in double quotes. Using "low" is correct.

4fill in blank

hard

Fill both blanks to create a factor f from vals and relevel it to have "second" as the reference level.

R Programming

vals <- c("first", "second", "third")
f <- factor([1])
f <- relevel(f, ref = [2])
levels(f)

Drag options to blanks, or click blank then click option'

Avals

B"first"

C"second"

Dc("first", "second", "third")

Attempts:

3 left

💡 Hint

Common Mistakes

Creating the factor from a wrong vector or literal.

Setting a reference level not present in the factor.

Use the vector vals to create the factor, then set the reference level to "second".

5fill in blank

hard

Fill all three blanks to create a factor f from data, relevel it to "C", and then get the levels.

R Programming

data <- c("A", "B", "C", "A")
f <- factor([1])
f <- relevel(f, ref = [2])
result <- levels([3])

Drag options to blanks, or click blank then click option'

Adata

B"C"

Cf

D"B"

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names in any of the steps.

Setting a reference level not in the factor.

Create the factor from data, set "C" as the reference level, and get the levels of f.

Practice

1. What does the relevel() function do in R when working with factors?

easy

A. It changes the reference level of a factor to a specified category.

B. It converts a factor into a numeric vector.

C. It sorts the levels of a factor alphabetically.

D. It removes unused levels from a factor.

Solution

1. Step 1: Understand the purpose of relevel()

   The relevel() function is used to set a new baseline or reference level for a factor in R.

2. Step 2: Identify what changing the reference level means

   Changing the reference level means selecting which category is treated as the baseline for comparisons or modeling.

3. Final Answer:

   It changes the reference level of a factor to a specified category. -> Option A

4. Quick Check:

   relevel() sets new baseline [OK]

Hint: Remember: relevel() sets the baseline category [OK]

Common Mistakes:

• Thinking relevel() converts factor to numeric
• Confusing relevel() with sorting levels
• Assuming relevel() removes unused levels

2. Which of the following is the correct syntax to set the reference level of a factor f to "Control" using relevel()?

easy

A. relevel(f, reference = "Control")

B. relevel(f, level = "Control")

C. relevel(f, base = "Control")

D. relevel(f, ref = "Control")

Solution

1. Step 1: Recall the argument name for reference level in relevel()

   The correct argument to specify the new reference level is ref.

2. Step 2: Match the syntax with the argument

   Only relevel(f, ref = "Control") uses the correct argument name and syntax.

3. Final Answer:

   relevel(f, ref = "Control") -> Option D

4. Quick Check:

   Argument for reference is ref = "Control" [OK]

Hint: Use ref= to set new baseline in relevel() [OK]

Common Mistakes:

• Using incorrect argument names like level or base
• Forgetting quotes around the reference level
• Using assignment instead of argument syntax

3. Given the code:

f <- factor(c("Low", "Medium", "High", "Low"))
f_new <- relevel(f, ref = "High")
levels(f_new)

What is the output of levels(f_new)?

medium

A. ["High", "Low", "Medium"]

B. ["Low", "Medium", "High"]

C. ["Medium", "Low", "High"]

D. ["High", "Medium", "Low"]

Solution

1. Step 1: Understand original factor levels

   Original levels are in alphabetical order: "High", "Low", "Medium".

2. Step 2: Effect of relevel(f, ref = "High")

   Relevel moves "High" to be the first (reference) level, keeping others in original order after it.

3. Final Answer:

   ["High", "Low", "Medium"] -> Option A

4. Quick Check:

   Reference level first = ["High", "Low", "Medium"] [OK]

Hint: Reference level appears first in levels() output [OK]

Common Mistakes:

• Assuming levels reorder alphabetically after relevel
• Confusing order of levels after relevel
• Expecting levels to be unchanged

4. Consider this code snippet:

f <- factor(c("A", "B", "C"))
f_new <- relevel(f, ref = "D")
levels(f_new)

What will happen when this code runs?

medium

A. ["A", "B", "C"] unchanged because "D" is ignored.

B. ["D", "A", "B", "C"] as new levels with "D" added.

C. An error because "D" is not a level in the factor.

D. Levels reordered alphabetically without error.

Solution

1. Step 1: Check if "D" is a level in factor f

   Levels are "A", "B", "C"; "D" is not present.

2. Step 2: Behavior of relevel() with invalid ref

   relevel() throws an error if the reference level is not in the factor levels.

3. Final Answer:

   An error because "D" is not a level in the factor. -> Option C

4. Quick Check:

   Invalid ref level causes error [OK]

Hint: Reference must exist in levels or error occurs [OK]

Common Mistakes:

• Assuming relevel adds new levels automatically
• Expecting no error and unchanged levels
• Thinking levels reorder alphabetically silently

5. You have a factor f with levels "Red", "Green", "Blue". You want to set "Green" as the reference level and then create a summary table counting each level's occurrences using the summary() function. Which code correctly does this?

hard

A. f_new <- relevel(f, ref = "Green")
table(f_new)

B. f_new <- relevel(f, ref = "Green")
summary(f_new)

C. f_new <- relevel(f, ref = "Green")
table(f)

D. f_new <- relevel(f, ref = "Green")
summary(f)

Solution

1. Step 1: Relevel factor to set "Green" as reference

   Use f_new <- relevel(f, ref = "Green") to create a new factor with "Green" first.

2. Step 2: Use summary() on the re-leveled factor

   Calling summary(f_new) counts occurrences of each level in the updated factor.

3. Final Answer:

   f_new <- relevel(f, ref = "Green") summary(f_new) -> Option B

4. Quick Check:

   summary() on re-leveled factor gives counts [OK]

Hint: Use summary() on re-leveled factor for counts [OK]

Common Mistakes:

• Using summary() on original factor after relevel
• Using table() on original factor instead of new
• Not assigning relevel result to new variable