While calculating the incubation period for measles in a group of 25 children, deviation is 2 and the mean incubation period is 8 days. Calculate the standard error -: (A) 0.4, (B) 1, (C) 0.5, (D) 2
Answer is A. STANDARD ERROR OF MEAN (SEMEAN)

SEmean is the measure of difference between sample and population values: Whatever be the sampling procedure or the care taken while selecting sample, the sample estimates (statistics) will differ from population values (parameters)
SE is a ‘measure of chance variation’, and IT DOESNOT mean an error or mistake.