Simple Random Sample How to make sure your sample is good... or hopefully good... maybe...

Definition The process of getting a simple random sample simply means that every unit in your sampling frame has an equal probability of being selected for your sample, every group of the same size has the same probability of being chosen, and you use some sort of randomness to select the units for your sample. This is a very common method of sample selection, employed primarily for its ease of implementation.

What does randomness mean? Anything where the outcome is unpredictable can be considered random. Examples: Flipping a fair coin, rolling a die, picking numbers out of a hat, picking a card from a well shuffled deck, etc. This is a great tool for sampling because it greatly reduces the bias involved in human sampling.

Why do we even sample in the first place? Sampling gives us a way to use a small group of individuals to make inferences about a whole population. If we get a good, or representative, sample of a population, our inferences on that population should be quite accurate. The question is, should we sample just one time, or should we do it more than once?

Sampling using computer software We can use computer software to help us simulate hundreds and hundreds of samples. Based on all of those samples, we can make a very good guess about the actual population. But in order to do this, we must be aware of what our response variable is. This is whatever information we want to get from our individuals in the sample.

Simulation Example Suppose we wanted to see if a random number generator was actually random. We could randomly sample 10 numbers from the generator at a time, and record how many of each we got. But using a computer, we could simulate thousands of samples of size 10, and record how many of each number we got in all those samples.

Benefits of simulation This allows us to look at many more samples than we would be able to get through by taking samples by hand. This allows us to look at many more samples than we would be able to get through by taking samples by hand. Many companies use this to get a good idea of certain demographics they are interested in. Many companies use this to get a good idea of certain demographics they are interested in. But back to the matter at hand... But back to the matter at hand...

How can I get a random sample? There are many common, and practical, ways that researchers can generate random numbers. There are many common, and practical, ways that researchers can generate random numbers. Microsoft Excel has a built in feature that will generate random numbers between two numbers that you decide, and you can just click and drag to get more random numbers. Microsoft Excel has a built in feature that will generate random numbers between two numbers that you decide, and you can just click and drag to get more random numbers. Many other computer programs will have built in functions like this. Many other computer programs will have built in functions like this.

Going old school One of the oldest methods is the use a random number or random digit table, which is just row after row after row of random numbers. This has been used for many, many years, and has proven to be very useful and effective for creating random samples. One of the oldest methods is the use a random number or random digit table, which is just row after row after row of random numbers. This has been used for many, many years, and has proven to be very useful and effective for creating random samples. However, with the advent of computers, using a random digit table to large samples would be very time consuming and not worthwhile. However, with the advent of computers, using a random digit table to large samples would be very time consuming and not worthwhile.

Random Number Table Here is an example of a random number table Here is an example of a random number table You can start wherever You can start wherever you want to on the table. you want to on the table. This helps with your This helps with your sample being random. sample being random.

Identifying your units To be able to use a random number table or random number generator, you need to have numbers assigned to each of your units in your sampling frame. To be able to use a random number table or random number generator, you need to have numbers assigned to each of your units in your sampling frame. It is usually easiest to start at 1,2,3,... all the way up to however many units there are in your sampling frame. It is usually easiest to start at 1,2,3,... all the way up to however many units there are in your sampling frame. This might be slightly different depending on what method you are using (random number table, random number generator, etc). This might be slightly different depending on what method you are using (random number table, random number generator, etc).

Easy example Say we have 5 units in our sample...well, go on, say it! Good. Say we have 5 units in our sample...well, go on, say it! Good. We would give each unit a number 1-5. We would give each unit a number 1-5. If we want a simple random sample of size 2, we can go to any row of a random number table, and pick the first two single digits that are between 1 and 5. If we want a simple random sample of size 2, we can go to any row of a random number table, and pick the first two single digits that are between 1 and 5. This can be extended to bigger sampling frames and sample sizes. This can be extended to bigger sampling frames and sample sizes.