Sampling

How to Select a Sample

Basic Concepts in Sampling

• Population: the entire group under study as defined by research objectives

–Researchers define populations in specific terms such as “heads of households

located in areas served by the company who are responsible for making the pest control

decision.”

• Sample: a subset of the population that should represent the entire group

• Sample unit: the basic level of investigation

• Census: an accounting of the complete population

Basic Concepts in Sampling

• Sampling error: any error in a survey that occurs because a sample is used

• A sample frame: a master list of the entire population

• Sample frame error: the degree to which the sample frame fails to account for all of

the population…a telephone book listing does not contain unlisted numbers

Probability Sampling:

Simple Random Sampling

• Simple random sampling: the probability of being selected into the sample is “known”

and equal for all members of the population

Ex. To select a sample of 5 from population of 100 people ranged from ID001 to

ID100. In other words, it is to select 5 random numbers from first number (ID001) to

the last number (ID100). The result on the table provided (5 numbers from 1 to 100)

shows that they are 3, 9, 65, 93, 96. Thus, the sample should include the members

of ID003, ID009, ID065, ID093, and ID096 based on the random number table

provided.

Systematic Sampling

• Systematic sampling: the way to select a random sample from a directory or list that

is much more efficient than simple random sampling

1. Find skip interval = population list size/sample size

2. Find the first member and then the others.

Ex. To select a sample of 5 from population of 100 people ranged from ID001 to

ID100. First of all, find the skip interval and in the case, it would be 20 because

100/5 = 20. Then, find the first member within the first interval from ID001 to ID020.

In this case, the first member should be ID015 based on the random number table

provided (one number from 1 to 20). Therefore, the rest of the sample should be

every 20th after the first one (ID015…ID035…ID055…). Thus, all the members of

the sample are ID015, ID035. ID055, ID075, and ID095.

Cluster Sampling

• Cluster sampling: the method in which the population is divided into groups, any of which can be considered a representative sample

Ex. To select a sample of 5 from population of 100 people from ID001 to ID100

(Group A: ID001-ID020, Group B: ID021-ID040, Group C: ID041-ID060, Group D:

ID061-ID080, Group E: ID081-ID100). First of all, select one group only to

represent the population. The group should be Group E based on the random

number table I gave you (one number from 1 to 5). Then, find the members within

the group. In this case, the members should be ID082, ID086, ID091, ID096, ID098

(5 numbers from 1 to 20) based on the random number table provided.

Stratified Sampling

• Stratified sampling: method in which the population is separated into different strata

and a sample is taken from each stratum

Ex. To select a sample of 5 from population of 100 people from ID001 to ID100

(Group A: ID001-ID020, Group B: ID021-ID040, Group C: ID041-ID060, Group D:

ID061-ID080, Group E: ID081-ID100). First of all, find how many members

needed for each group. In this case, there is only one member needed for each

group because 5/5 = 1. Then, find which orders for each group. In this case, the

member to be selected should be always the fifteenth in each group. Therefore,

ID015, ID035, ID055, ID075, ID095 are the answers based on the random

number table provided (one number from 1 to 20).