Systematic Sampling

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.

In a random sample every member of the population has an equal chance of being chosen, which is clearly not the case here, but in practice a systematic sample is almost always acceptable as being random.

Advantages

• spreads the sample more evenly over the population

• easier to conduct than a simple random sample

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