Author:
Al Greene

Demonstrate a working knowledge of randomness using examples whenever possible

Show how to use SRS as a technique to gather data

This packet introduces you to simple random sampling, a basic method of sampling. There are definitions, simple examples, somewhat more complicated examples, and reasoning behind why we use this method.

Tutorial

This packet introduces you to the concept of simple random sampling. We will define simple random sampling, show why it is used, how people use it, and illustrate some examples. There is a very simple example in the powerpoint, and more in depth examples in the videos below.

Source: Greene

This video shows how to use a random number generator to pick a random sample. This a very easy to use tool to find a random sample of numbers.

Source: YouTube

This video talks about how to take a random sample from a population.

Source: YouTube

There are some finer points to mention in dealing with the random number table.

If you are using different size populations, you will have to include a certain amount of digits when labeling your units.

Specifically, your labels must have as many digits as the number of units that you have in your population.

For example, if you have a population of size 75, every label must have two digits. It would look like this:

01, 02, 03, 04, etc.

This is because you must look at two digits every time to ensure that every unit has a chance to be selected.

If you had a population of size 4000, you would have to have 4 digits on each label, or:

0001,0002,0003,...,3997,3998,3999,4000.

Whatever the number of units in your population, you have to look at that many digits every time in your random number table.

Also, what happens if you get the same number twice?

If you are sampling with replacement, this is fine. But, like most studies, if you are only using each unit a maximum of one time, you will skip over that number and go onto the next.

For example, if you have a population of size 200, your labels would look like this:

001,002,003,...,198,199,200.

Suppose we want a sample of 10 units. Our first three units chosen were 023,178, and 055. If we were to come across another group of three digits that were 023, we would skip over all three of them, and go to the next set of three digits.

Try this out if you are having difficulty with this concept.

Source: Greene