What is the independent variable in this experiment?

This is just an example, but if you were to preform an experiment to see if there is a correlation between leg length and running speed, what is the independent variable? Normally it's the variable that is manipulated, but in this case, none of the variables are manipulated.


Thanks for the help!|||leg length





and yes it can be manipulated. The researcher can pick specific individuals based on their leg lengths (individuals with 20" leg length, individuals with 30'' leg length, individuals with 40'' leg length). Instead of using the word manipulated, think of it as a variable in which the researcher changes on purpose.|||The manipulation comes with you selecting the person(s) to be measured, For example, you might select a person with short legs, one with medium legs and another with long legs.|||Your avatar looks like my avatar after it took a bunch of ecstasy.