To assign to or create a variable in a table, the number of rows must match the height of the table.

Answer 1

[Walter Roberson]

Grades{:, 2:4} is going to be a something by 3 array. mean() by default operates on the first nonsingular dimension so that the output would be 1x3. I suspect that you want to add the parameter ,2 to the mean call so that it takes the mean along rows

mean(something, 2)