Chapter 3 Sampling Techniques

Chapter 3 – Sampling Techniques When conducting a survey, it is important to choose the right questions to ask and to select the appropriate group to survey.

It is usually impractical to survey every member of a target group or population, so only a sample of the population is surveyed.

The population refers to all individuals or items that belong to a group being studied.

The sample refers to a group of individuals or items that are representative of the population from which they are taken.

For example, prior to an election it is impossible to predict the result of the election by asking EVERY person in the country who they’ll vote for.

Therefore, a sample of the population is chosen, and each of the people in that sample are asked who they intend to vote for.

Bias Bias

Bias It’s important to make sure that the sample is representative, otherwise we could get biased results It’s important to make sure that the sample is representative, otherwise we could get biased results

That’s what happened when the Chicago Daily Tribune reported that Dewey defeated Truman in That’s what happened when the Chicago Daily Tribune reported that Dewey defeated Truman in 1948.

A survey contains bias if it does not reflect the population. A survey contains bias if it does not reflect the population.

Bias may be caused by Bias may be caused by 1) an unrepresentative sample, or 2) the wording of the survey questions, and/or 3) the interpretation and presentation of the results.

Let’s talk about an unrepresentative sample first.

Why would someone perform a survey and choose a sample that is not representative? Why would someone perform a survey and choose a sample that is not representative?

2 main reasons

Time Time Time flies

and

Money Money

Sometimes there just isn’t enough time or money to do everything in the best possible way. Sometimes there just isn’t enough time or money to do everything in the best possible way.

Various Sampling Techniques

Here are some examples of samples that may be used.

1. Simple Random Sample  In a simple random sample, each member of the population has an equal chance of being selected. →

Example  Suppose the principal wants to determine how different teachers at Massey feel about a certain issue. She doesn’t have time to interview all 90 teachers, so instead she puts all 90 names in a hat, and draws some of those names, and then asks each of them.

Pros and Cons  What are advantages of using a simple random sample?  What are disadvantages?

2. Stratified Random Sample ► In a stratified random sample, the population is divided into subgroups (for example, by gender, age, nationality), and a random sample is selected from each subgroup in proportion to its size in the population. →

Example ► Go back to our last example. Suppose the principal wants to stratify her sample, so that she doesn’t get all men, or all women, or all old teachers, or all young teachers, etc. One thing she could do is … →

► She could divide all the teachers in the school into four categories 1.Newer female teachers 2.Newer male teachers 3.More experienced female teachers 4.More experienced male teachers. →

► Then, rather than putting the names in one big hat, the principal could put the names in four little hats. Then, she could take one quarter of the names that she needs from each hat.

Pros and Cons ► What are advantages of using a stratified random sample? ► What are disadvantages?

3. Voluntary-Response Sample In a voluntary-response sample, the sample contains those members of the population who have chosen to respond to the survey. → In a voluntary-response sample, the sample contains those members of the population who have chosen to respond to the survey. →

Example The principal could put some blank response sheets on the table in the staff room with an invitation to all teachers to return them to her. Then, hopefully, the responses that she gets from the teachers that choose to respond will be representative of the feelings of the whole group. The principal could put some blank response sheets on the table in the staff room with an invitation to all teachers to return them to her. Then, hopefully, the responses that she gets from the teachers that choose to respond will be representative of the feelings of the whole group.

Pros and Cons What are advantages of a voluntary- response sample? What are advantages of a voluntary- response sample? What are disadvantages? What are disadvantages?

4. Cluster Sample In a cluster sample, the population is divided into clusters and a certain number of clusters are chosen. Every member of these clusters is part of the sample.→

Example Suppose the principal decided that she would wait until a P.A. Day. Then, when the departments were having their meetings, she would go to three of the departments and ask what the teachers think of the issue. For example, she might go to the science department, the family studies department, and the phys. ed. department.

Pros and Cons What are advantages of a cluster sample? What are disadvantages?

5. Convenience Sample In a convenience sample, the sample contains those members of the population from which data are most easily collected.→

Example Perhaps the principal will ask the first ten teachers that she sees at school the next day what they think of the issue.

Pros and Cons What are advantages of a convenience sample? What are disadvantages?

6. Systematic Sample In a systematic sample, every nth member of the population is selected. In a systematic sample, every nth member of the population is selected.→

Example Perhaps the principal will list all the teachers in alphabetical order, then ask the 10 th teacher on the list, the 20 th teacher, the 30 th teacher, and so on until she was done. Perhaps the principal will list all the teachers in alphabetical order, then ask the 10 th teacher on the list, the 20 th teacher, the 30 th teacher, and so on until she was done.

Pros and Cons What are advantages of a systematic sample? What are advantages of a systematic sample? What are disadvantages? What are disadvantages?