Odds would suggest that if a value appears in a request to the server that you have not explicitly selected or typed into an interface, then these "alien" values would need to be correlated. Many techniques are available. They are covered both in training and in the application documentation.
Read your Mercury license carefully, for if you do not have explict permission to be testing against Yahoo, then you are violating your license agreement. Yahoo may also have grounds to come against you financially if you produce enough load to impact their line of business operations. This is why the sample applications exist.