An experimenter publishing in the Annals of Botany investigated whether the stem diameters of the dicot sunflower would change depending on whether the plant was left to sway freely in the wind or was artificially supported. The average diameter of unsupported stem at the base of a particular species of sunflower plant is 36 mm with standard deviation of 9.1 mm. Suppose that a random sample 84 plants are selected from the said species of sunflowers, what would be the standard deviation of the sampling distribution of the sample average diameter