Define (a) standard deviation, (b) standard error of the mean and (c) 95% confidence interval

Standard deviation (SD) is a statistical measure of the spread of a series of values. SD is the square root of variance. In a given population SD varies little with sample size. In a normally distributed population 66%of values lie within 1 SD of the mean and 95%within 1.96 SD. The smaller the SD the more closely grouped the values.