Refer to Exercise 50. Assume that the number of nonword errors X and word errors Y in a randomly selected essay are independent random variables. Calculate and interpret the standard deviation of the sum S = X + Y.

Exercise 50.

Typographical and spelling errors can be either “nonword errors” or “word errors.” A nonword error is not a real word, as when “the” is typed as “teh.” A word error is a real word, but not the right word, as when “lose” is typed as “loose.” When students are asked to write a 250-word essay (without spell-checking), the number of nonword errors X in a randomly selected essay has mean 2.1 and standard deviation 1.136. The number of word errors Y in the essay has mean 1.0 and standard deviation 1.0. Calculate and interpret the mean of the sum S = X + Y.