Two Different Questions: What Is Described, and What Is Recorded?
In Common Mistakes Classifying Variable Types, you practiced separating the observational unit from a variable. This tutorial focuses on a frequent mix-up: answering “what information was recorded?” when asked “what does one case represent?”—or answering in the opposite direction.
The observational unit is the person, object, group, or event represented by one case. A variable is a characteristic recorded about each unit. The unit is what the data describe; the variable is what the data say about it. These answers are related, but they do different jobs.
For example, if a data table has one row for each bicycle, “one bicycle” names the observational unit. “Bicycle weight in kilograms” names a variable. Weight is measured, but it is not the entity represented by a row. If asked for the variable, answering “bicycles” would make the opposite mistake.
This sentence is a useful new check because it makes the relationship between the answers explicit. If the sentence sounds as if a measurement or label were itself the person or object, reconsider your wording. The unit and variable should fit together as “something described” and “information about it.”
Use the Case-by-Case Test
A data table gives you a practical clue: read across one row and ask, “What single case is all this information about?” Then read the columns and ask, “What characteristic does each column record for that case?” As explained in Reading a Data Table: Rows, Columns, and Cases, rows represent cases and columns record variables.
A second check is to ask what changes from row to row. If a row gives a participant’s name, age, and preferred activity, the participant is the unit. Age and preferred activity are variables. The values can differ across participants, but the participant is the entity to which those values belong.
Do not let a column heading take the place of the case. A table with columns for “distance,” “duration,” and “route type” does not necessarily have distances, durations, or routes as its units. You still need to know what one row represents. It might be one trip, with distance, duration, and route type recorded for each trip.
Use the row description, table structure, or study description to determine what one record represents.
State the person, object, group, or event represented by that case, as specifically as the context allows.
List the relevant characteristic or characteristics. These are variables, not the unit itself.
Say, “For each [unit], the data record [variable].” Check that the two phrases do not answer the same question.
Worked Examples: Correct the Mix-Up
Worked Example: What Does Each Garden Row Describe?
A fictional city inventory has one row for each community garden. The columns list its neighborhood, number of plots, and whether it has a tool shed. A student is asked to name the observational unit and answers, “The number of plots.” Correct the answer.
Find one case: The description says that each row represents one community garden. The observational unit is therefore one community garden.
Identify the mix-up: “Number of plots” is a characteristic recorded for each garden. It is a variable, not the entity represented by a row. The fact that plot count is a number does not make it the unit.
Corrected answer: “The observational unit is one community garden. For each garden, the data record its neighborhood, number of plots, and tool-shed status.”
Why the full sentence helps: “One community garden” answers what each case represents. The three listed fields answer what is recorded about that case. Naming both also confirms that the unit is specific enough to fit the table.
Worked Example: The Respondent Is Not Always the Unit
A fictional family-services survey is completed by one adult in each household. Each adult reports the number of children in the household and whether the household has a regular bedtime routine. The data file has one row per responding adult. A student says, “The unit is the children, because the survey asks about them.” Is that correct?
Find what one row represents: The data file has one row per responding adult. The observational unit in this data set is one responding adult, with one adult representing a household in the survey.
Separate the respondent from the information: Children are mentioned in a question, but the data are not described as having one row for each child. “Number of children in the household” is a variable reported for each adult’s household. Bedtime-routine status is another variable.
Corrected answer: “The observational unit is one responding adult. For each adult, the data record the number of children in the adult’s household and whether the household has a regular bedtime routine.”
Important qualification: If a different data file had one row for every child and recorded each child’s bedtime routine, then the child could be the observational unit. The topic of a question alone does not settle the unit; the case structure does.
Worked Example: Trips, Not Travelers
A fictional bike-share report contains one row for each completed trip. Its columns record the trip’s start station, duration in minutes, and distance in kilometers. A student is asked to name the unit and answers, “Distance in kilometers.” Give a more accurate answer.
Find one case: The report states that a row represents one completed trip. The observational unit is one completed bike-share trip.
Identify the variable: Distance in kilometers is a quantitative variable recorded for each trip. Duration and start station are also variables. Neither the measured distance nor the starting location is the trip itself.
Corrected answer: “The observational unit is one completed bike-share trip. For each trip, the data record its start station, duration, and distance.”
Why not answer ‘bicycle’? A bicycle is involved, but the stated case is a completed trip. One bicycle can be used on multiple trips. Unless each row represents a bicycle, naming “bicycle” would replace the actual unit with a different entity.
Worked Example: When Each Case Is an Occasion
A fictional school tracks the number of minutes a student spends reading during each assigned weekly reading session. The data file has one row for each student-session combination. A student says, “The unit is a student.” Is that precise enough?
Read the row description carefully: A row represents one student-session combination, not just one student across the whole term. A student may have several rows, one for each recorded session.
Name the unit and variable: The observational unit is one student’s reading session. The variable is the number of minutes that student spent reading during that session.
Corrected answer: “The observational unit is one student-session. For each session, the data record the student’s reading time in minutes.”
Why this distinction matters: “Student” gives part of the description, but leaves out which occasion the row represents. Adding “session” makes the unit match the data structure. In another data set with one row per student and a single term-long total, the unit could instead be one student. Name the unit that the actual case definition supports.
Compare the Answers Side by Side
The following table summarizes the difference in several settings. In each row, the unit answers what a case is, while the variable answers what is recorded for that case.
| What one row represents | Example variable | Common mix-up |
|---|---|---|
| One community garden | Number of plots | Calling plot count the unit |
| One responding adult | Number of children in the household | Calling the children the unit because they are mentioned |
| One completed bike-share trip | Distance in kilometers | Calling distance the unit |
| One student-session | Reading time in minutes | Naming only the student and leaving out the occasion |
These examples also show why there is no universal answer to “What is the unit?” The answer depends on how the cases are defined in that particular data set. A study can record a person’s experience while using a session as the unit, or it can record information supplied by one person about a household. State what each row represents rather than guessing from the subject area.
Common Mistakes and AP Exam Tips
- Answering a unit question with a measurement. “Time in minutes,” “number of plots,” and “distance” are variables. Name the object or event being described by one case.
- Answering a variable question with a group of people. “Students” may be the unit, but it does not tell the reader what information was recorded. Add the characteristic, such as each student’s usual travel method.
- Choosing the subject mentioned most often. A question about children does not establish that children are the units. Check the row definition and who or what has one record per case.
- Assuming the physical object is always the unit. If rows represent trips made with bicycles, the trips—not necessarily the bicycles—are the units. Follow the case definition.
- Giving a unit that is too broad. “Student” may miss an important occasion when each row represents one student-session. Include the detail that distinguishes one case from another.
A full-credit answer should directly name the requested item and use context. If asked for both, state them separately or use the sentence frame: “For each [unit], the data record [variable].” For instance: “For each completed trip, the data record distance in kilometers.” “Trip” names the unit; “distance in kilometers” names the variable. Do not rely on a bare word such as “distance” when the question asks for the observational unit.
Check Your Understanding
For each situation, identify the observational unit and one variable. If the student’s answer is incorrect, explain the mix-up and correct it.
- A park inventory has one row per playground and records the number of swings. A student says the unit is “number of swings.” What are the unit and variable?
- A survey file has one row per caregiver. Caregivers report the average number of hours per child in their household spent on outdoor activities, and this average is recorded for each caregiver. What is the unit, and what is one recorded variable? What information would you need to know to decide whether each child also has a separate case?
- A delivery company records one row per delivery and includes the delivery time in minutes. A student names the driver as the unit. What does the stated row structure support as the unit, and what is the variable?
- A school records one row for each student on each test date, along with the student’s score. Why might “one student” be less precise than the unit given by the row structure?
- Write a complete sentence naming the unit and one variable for a data set with one row per house and a column recording the number of exterior doors.