This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
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 - 
Select and explain. 
A. 0.4, B. 1, C. 0.5, D. 2
A: 0.4
Answer: A: 0.4 
Explanation: 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.