This dichotomy is artificial. Notation is r with b subscript. Phi is when each of the varibles are naturally occuring, dichtomies. Ex. - gender with handedness. Notation is a circle with a line through it vertically.
Tetrachoric correlation is two artificially dichotomous variables.
I then gave you a handout with an assignment. That handout is called Pearson.r. Get handout called Pearson.r. Click here to return to the top of the page.