EXAMPLE 2 Length, college readiness, highway safety

To measure the length of a bed, you can use a tape measure as the instrument. You can choose either inches or centimeters as the unit of measurement. If you choose centimeters, your variable is the length of the bed in centimeters.

To measure a student’s readiness for college, you might ask the student to take the SAT Reasoning exam. The exam is the instrument. The variable is the student’s score in points, somewhere between 400 and 1600 if you combine the Evidence-Based Reading and Writing and Mathematics sections of the SAT. “Points” are the units of measurement, but these are determined by a complicated scoring system described at the SAT website (www.collegeboard.com).

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How can you measure the safety of traveling on the highway? You might decide to use the number of people who die in motor vehicle accidents in a year as a variable to measure highway safety. The government’s Fatality Analysis Reporting System collects data on all fatal traffic crashes. The unit of measurement is the number of people who died, and the Fatality Analysis Reporting System serves as our measuring instrument.