Random sampling: systematic sampling

Written by Carlos Ochoa el 23 de January 2017

Now that we have taken a look at stratified sampling, we’d like to continue our series of posts on sampling techniques by examining systematic sampling.

Systematic sampling used to be very popular, before computer science solved one of researchers’ biggest headaches: randomly selecting individuals from within a sample. Now that computers can generate random numbers, this problem has vanished.

It is still used, however, to select individuals over a period of time. For example, if we want to study satisfaction with a service at a store, we can systematically survey one out of ever n customers who visit. In these cases, in which the variance between individuals could be different at different moments, systematic sampling can be even more precise than pure simple random sampling.


Category: online sample | random sampling | sampling

Random sampling: stratified sampling

Written by Carlos Ochoa el 18 de January 2017

In an earlier post, we saw the definition, advantages and drawback of simple random sampling. Today, we’re going to take a look at stratified sampling.

This method, which is a form of random sampling, consists of dividing the entire population being studied into different subgroups or discrete strata (the plural form of the word), so that an individual can belong to only one stratum (the singular). Once the strata have been defined, in order to create a sample, we select individuals by applying a sampling method to each of the strata separately. If, for example, we use simple random sampling for every stratum, we’re using what’s called stratified random sampling (StratRS). We can also use other sampling methods for every stratum, such as systematic sampling and random sampling with replacement.

Strata tend to be homogeneous groups of individuals, while groups are heterogeneous among themselves. For example, if we’re expecting very different behavior between men and women in a study, it would be convenient to define two strata, one for each gender. If we have selected these strata correctly, (1) men should behave similarly to each other, (2) women should behave similarly to each other and (3) men and women should exhibit dissimilar behavior.

Category: online sample | random sampling | sampling

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