Compute the similarity between 2 data sets

I have 2 data sets of an experiment. One is from VNA spectrum analyzer and other is from PRBS analyzer. I get 2 graphs from those data sets which are look alike almost same. But for my project purpose i have to get the similarity between those 2 graphs mathematically. How can I obtain that the graphs are similar or not or the similarity percentage?

 And plz help me to to get the similarity percentage if the graphs are not similar.

Answer 1

[Aquatris]

Here is a nice reference on FRF correlation techniques. Go to section 2 of the article, where you will find a couple of methods.