RSQ function
Returns the square of the Pearson product moment correlation coefficient through data points in known_y's and known_x's. For more information, see PEARSON. The rsquared value can be interpreted as the proportion of the variance in y attributable to the variance in x.
Syntax
RSQ(known_y's,known_x's)
Known_y's is an array or range of data points.
Known_x's is an array or range of data points.
Remarks

Arguments can either be numbers or names, arrays, or references that contain numbers.

Logical values and text representations of numbers that you type directly into the list of arguments are counted.

If an array or reference argument contains text, logical values, or empty cells, those values are ignored; however, cells with the value zero are included.

Arguments that are error values or text that cannot be translated into numbers cause errors.

If known_y's and known_x's are empty or have a different number of data points, RSQ returns the #N/A error value.

If known_y's and known_x's contain only 1 data point, RSQ returns the #DIV/0! error value.

The equation for the Pearson product moment correlation coefficient, r, is:
where x and y are the sample means AVERAGE(known_x’s) and AVERAGE(known_y’s).
RSQ returns r2, which is the square of this correlation coefficient.
Example
The example may be easier to understand if you copy it to a blank worksheet.
How to copy an example

Create a blank workbook or worksheet.

Select the example in the Help topic.
Note Do not select the row or column headers.
Selecting an example from Help

Press CTRL+C.

In the worksheet, select cell A1, and press CTRL+V.

To switch between viewing the results and viewing the formulas that return the results, press CTRL+` (grave accent), or on the Formulas tab, in the Formula Auditing group, click the Show Formulas button.

