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Random sampling: simple random sampling

Written by Carlos Ochoa el 16 de January 2017

Continuing with our series of posts on sampling, today we'll review the first random sampling method: simple random sampling. This is one of the most popular sampling methods, and it serves as a reference for many others, even though, as we’ve said before, in practice it can be difficult to implement.

DEFINITION

Simple random sampling (SRS) is a sampling method in which all of the elements in the population—and, consequently, all of the units in the sampling frame—have the same probability of being selected for the sample. It would be along the lines of having a fair raffle among every individual in the population: we give everyone raffle tickets with unique sequential numbers, put them all in a basket and draw numbers from the basket at random. The individuals whose numbers are selected become our sample. Obviously, in practice, these methods can be automated using computers.

Category: online sample | random sampling | sampling

Random and non-random sampling

Written by Carlos Ochoa el 12 de January 2017

In a recent post, we learned about sampling and the advantages it offers when we want to study a population. Today, we're going to take a look at the two main sampling methods. Let’s start by defining the concept of a sampling frame.

SAMPLING FRAME

A sampling frame is a list of elements that make up the population that we want to study. The sample is drawn from this list. The elements to be studied could be individuals, but they could also be households, institutions or anything else that might be investigated. The elements within the sampling frame are known as sampling units

 

 

Let’s look at an example. Suppose we want to gauge customers’ satisfaction with a particular business. To create our sampling frame, we could access the business’s computer system and pull up a list of everyone who has purchased a product in the past year. Every individual on that list would be considered a sampling unit. We could then select our sample by choosing a group of these customers.

The proportion of the sampling frame included in the sample is known as the sampling fraction. We saw in an earlier post, that this fraction, along with the sample size, determines the precision of the results that we will obtain by surveying our sample.

Category: random sampling | sampling

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