In some instances the most practical way of sampling is to select
every 15th name on a list, every 10th house on one side of a street and so on. Sampling of
this type is known as systematic sampling. An element of randomness is usually introduced
into this kind of sampling by using random numbers to pick up the unit with which to start.
This procedure is useful when sampling frame is available in the form of a list. In such a
design the selection process starts by picking some random point in the list and then every
nth element is selected until the desired number is secured.