The number of years of education of all self-employed individuals in the united states has a mean of 12. 8 years and a standard deviation of 2. 8 years. If we survey a random sample of 100 self-employed people, the average number of years of education for the sample is 13. 6 years, and the sample standard deviation s =3 years. What is the standard deviation of the sampling distribution of the sample mean?.