A few notes:
1. Make a habit of not using i or j as variables. They are the built-in imaginary unit. Overwriting them works, but can occasionally errors that are tough to debug.
2. Don't use sum as the name of a variable for a similar reason. It's a built-in function.
At the top of your example can you create some dummy variables that we can use to run your code? For example, this could be accomplished by
Rates = rand(10,30);
if Rates is a 10x30 matrix.
What's the goal of the code above? Can you describe the calculation you're trying to perform?
