I have been recently interested in the correlation factor function.
The function exists in Excel called "CORREL()". I have provided an Excel spread-sheet for examination of how this factor works. The CORREL function returns a value between 1 and -1 inclusive. In short, the values mean this:
This is illustrated below, correlation factor is based upon two datasets and reflects how closely they match. One word of caution, the data can be scaled and still correlate.
You will see in the above graphic 4 data series. Series 1 & 2 100% correlate although one is steeper than the other. Series 2 & 3 exactly match, but in reverse -100% correlation (ultimately same line only multiplied by -1). This also means Series 1 & 3 are exact reverse matches at -100%. Series 3&4 have very little correlation, a little reversed but ultimately no correlation 0%. Similarly Series 1&4 have almost no correlation a little positive.
The Excel sheet is included so you can experiment with this function and get a feel for how it works. The most shocking is the Series 1 & 2. I have played with determining a scale between these two so you can tell if they correlate and then how far they are different.