One of the most common errors found in the media is the confusion between correlation and causation in scientific and health-related studies. In theory, these are easy to distinguish — an action or occurrence can cause another (such as smoking causes lung cancer), or it can correlate with another (such as smoking is correlated with alcoholism). If one action causes another, then they are most certainly correlated. But just because two things occur together does not mean that one caused the other, even if it seems to make sense.

One way to get a general idea about whether or not two variables are related is to plot them on a “scatter plot”. If the dots on the scatter plot tend to go from the lower left to the upper right it means that as one variable goes up the other variable tends to go up also. This is a called a “direct (or positive) relationship.” On the other hand, if the dots on the scatter plot tend to go from the upper left corner to the lower right corner of the scatter plot, it means that as values on one variable go up values on the other variable go down. This is called an “indirect (or negative) relationship."

Using Excel 2016 to calculate the Correlation Coefficient

• ​Enter your data into two adjacent columns

• Click on the box in which you want the correlation coefficient to be placed

• Click the "Formulas" tab at the top of the screen

• Select the “Insert Function button”​

• Search for "correlation" and then select the PEARSON option. Click OK.

• Select one data set column for ARRAY1 and the other data set column for ARRAY2. Click OK.

• Once you have the correlation coefficient, you need to make sure that you set the values to the correct number of digits. EXCEL will default to giving you too many numbers after the decimal place. Your correlation should have the same precision (number of digits after the decimal) as your data points. If your data is a whole number, the correlation should be rounded at the tenths place. To adjust the number of digits in Excel, click the box which is displaying the correlation coefficient and on the "Home" tab click the decrease decimal button until you have the correct number of digits showing.