6.3 The central limit theorem for sums

Given a random variable (RV) with known mean μ and known standard deviation σ . We are sampling with size n and we are interested in two new RVs - the sample mean, X ¯ , and the sample sum, Σ X . If the size n of the sample is sufficiently large, then X ¯ ∼ N μ σ n and Σ X ∼ N ( nμ , n σ ) . If the size n of the sample is sufficiently large, then the distribution of the sample means and the distribution of the sample sums will approximate a normal distribution regardless of the shape of the population. The mean of the sample means will equal the population mean and the mean of the sample sums will equal n times the population mean. The standard deviation of the distribution of the sample means, σ n , is called the standard error of the mean.