Definition-
For example,
Suppose you want to sample 8 houses from a street of 120 houses.
120/8=15, so every 15th house is chosen after a random starting point between 1 and 15. If the random starting point is 11, then the houses selected are 11, 26, 41, 56, 71, 86, 101, and 116. If there were 125 houses, 125/8=15.625, so should you take every 15th house or every 16th house? If you take every 16th house, 8*16=128 so there is a risk that the last house chosen does not exist. To overcome this the random starting point should be between 1 and 10. On the other hand if you take every 15th house, 8*15=120 so the last five houses will never be selected. The random starting point should now be between 1 and 20 to ensure that every house has some chance of being selected.
- Sample over extended periods of time.
- Reduce the impact of a variety of conditions to prevent systematic errors from influencing the analysis of the data.
- One reason for this type of sampling is to see if the process has any variations due to different times of the day, shift workers changing, temperature changes during day vs. night times, etc.
Advantages.
Spreads the sample more evenly over the population
easier to conduct than a simple random sample
Easier, faster and less chance of making a sampling mistake, compared with SRS.
Could give a more precise estimate than SRS.
Disadvantages
the system may interact with some hidden pattern in the population, e.g. every third house along the street might always be the middle one of a terrace of three
Could be less precise than SRS
Effect of periodicity (bias caused by particular characteristics arising in the sampling frame at regular units). An example of this would occur if you used a sampling frame of adult residents in an area composed of predominantly couples or young families. If this list was arranged: Husband / Wife / Husband / Wife etc. and if every tenth person was to be interviewed, there would be an increased chance of males being selected.