The standard deviation of the observations 5, 5, 5, 5, 5 is
(a) 0 (b) 5 (c) 20 (d) 25

• Standard deviation is nothing but the square root of variance.
  And variance of a series will be defined as the extend of variation among it's members I.e. how much one observation differ from another.
  The above series has 5 members and they all have the same magnitude I.e.5
  This implies that there is no variation in the series. Hence variance should be 0.
  As I described above, S.D=sq.root of var.
  =Sq.root of 0
  S.D=0
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