A systematic sample does not yield good result, if -

a 1 in x systematic random sample is obtained by randomly selecting one of the first "x" units in the sampling frame and then selecting every subsequent xth unit Its generally used in a continuous process. First observation is chosen randomly and then other observations are picked by adding some constant "c" to the successive observations. c = N/n where N = size of population and n = size of sample It's important to check that the data is not having a periodic property. (Foe eg. If sales are analysed and x is taken as 7, then sample will include the sales on the same day of every week)