mean is a built-in function. It requires an argument, like mean(RF). Then it should work.
mu = mean(RF);
standarDeviation = std(RF);
deflectionAbove = RF > mu + standarDeviation % Logical vector
deflectionBelow = RF < mean - standarDeviation
deflection = deflectionAbove | deflectionBelow; % A logical vector
deflection = find(deflectionAbove | deflectionBelow); % A linear index vector
