One might wonder if the time spent studying for a test affects test score. These two variables would probably be positively related. As time studying for a test goes up, test score probably goes up too.
Click on image for full size
Courtesy of Freeze Clip Art

Correlations in Science

In conducting their research, scientists often want to know if two sets of data (variables) are related to each other. For instance, you might wonder if the amount of time a student spends reading the Windows to the Universe website is related to the grade that student gets in his or her science classes. How would you test this, and how would you express it in a way that would clearly tell other people what sort of relationship there is between these two variables?

One of the most common ways a scientist does this is by using a concept called correlation. Correlation is basically a measurement of how independent two different variables are, and is usually calculated using a formula that results in a coefficient of correlation ranging from -1 to 1.

A correlation of -1 indicates that the two variables are inversely related, and that as one variable increases the other always decreases. For example, the total sales in a given day for an ice cream truck and the total snowfall for that same day might have a correlation close to -1. On days with lots of snow, not many people are buying ice cream from the truck, and on days where the ice cream truckís sales are really high, itís probably not snowing. A correlation of 1, on the other hand, indicates that the two variables are directly related, and that as one variable goes up the other does also. For example, the amount of time a basketball player spends practicing is usually closely related to the number of points he or she scores in games, and this relationship would probably have a correlation coefficient close to 1.

Many times a calculated correlation will be close to 0, and this indicates that there is no obvious relationship between the two variables (there may still be a relationship; in some rare cases two variables can be closely related but have a correlation coefficient of 0). Itís important to remember that even when two variables are correlated, this does not mean that a change in one variable causes the other one to changeóit just means that theyíre related. For instance, when itís raining you can see people using umbrellas a lot more often, and you can see cars using their wipers a lot more often. So umbrella use and windshield wiper use are correlated, but neither causes the otherówe donít use umbrellas because other people are using wipers, or vice versa. We use both because itís raining.

Last modified January 25, 2008 by Jennifer Bergman.

You might also be interested in:

Sun's Effect on Earth's Weather (Wind)

Energy from the Sun is one of the primary drivers of the Earth system. The Sun warms our planet, heating the atmosphere. This energy feeds atmospheric processes and is a primary driver of our weather....more

A Trip to the Observatory

Have you ever wondered what it would be like to take a trip to an observatory to use the telescope? In February 2006, astronomer Travis Metcalfe was granted 7 nights of observing time on one of the telescopes...more

A Trip to the Observatory - Transcript

I'm Travis Metcalfe, an astronomer at the National Center for Atmospheric Research in Boulder, Colorado. I made this short video for anyone who's ever wondered what it's like to take a trip to the observatory...more

Correlations in Science

In conducting their research, scientists often want to know if two sets of data (variables) are related to each other. For instance, you might wonder if the amount of time a student spends reading the...more


Hipparchus was a Greek astronomer who lived between 190-120 B.C. He created the first accurate star map and kept a catalogue of over 850 stars with their relative magnitudes. The system of epicycles describing...more

James Adamson

James Adamson is an American astronaut who was born in New York in 1946. He attended military school from 1965-1969 and studied engineering. From 1969-1980 he stayed in the Army as an aviator and professor....more

Tom Akers

Tom Akers is an American astronaut who was born on May 20, 1951 in Missouri. Before he was an astronaut, Akers was a park ranger, teacher, and Air Force pilot. He has flown 25 different types of aircraft....more

Joseph Allen

Joseph Allen is an American astronaut who was born on June 27, 1937 in Indiana. Before he became an astronaut, Allen was a physicist who taught at the University of Washington. Allen became an astronaut...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA