Methods of ResearchLesson 6: Sample and Sampling

Dr. Racidon P. Bernarte

Sampling is a process of systematically selecting cases for a research project.

Sampling is that part of statistical practice concerned with the selection of individual observation intended to yield some knowledge about a population of concern, esp. for the purposes of statistical influence.

The process of selecting representative portion of the population.

What is Sampling?

The Concept of Sampling

• Census involves a complete enumeration of the elements of a population.

• Sample the sub group of the population selected for participation of the study.

Population is the aggregate of all elements, sharing some common set of characteristics

Census vs. Sample

Sampling

Who is the target

group for the study?

This is called the

study population

Who in the target

group should be

surveyed?

This is called the

sample.

How many people

should be surveyed?

This is called the

sample size.

How should the people

to be surveyed by

selected?

This is called the

sampling method.

1. Define the target population

2. Determine the sampling frame

3. Select a sampling technique(s)

4. Determine the sample size

5. Execute the sampling process

The Sampling Design process

Sampling Design Process

Sampling Process

Defining the

population

Developing

a sampling

Frame

Determining

Sample

Size

Specifying

Sample

Method

SELECTING THE SAMPLE

• Probability Sampling—sampling based on probability theory; this type of sampling yields a sample that is truly a representative of the population.

• Non-Probability Sampling—sampling that does not follow the guidelines of mathematical probability

Probability Vs. Non-Probability

i. Nonprobability Sampling Techniques

a. Convenience Sampling

b. Judgmental Sampling

c. Quota Sampling

d. Snowball Sampling

ii. Probability Sampling Techniques

a. Simple Random Sampling

b. Systematic Sampling

c. Stratified Sampling

d. Cluster Sampling

e. Other Probability Sampling Techniques

The Sampling Technique

Classification of Sampling Techniques

Sampling Techniques

Nonprobability

Sampling Techniques

Probability

Sampling Techniques

Convenience

Sampling

Judgmental

Sampling

Quota

Sampling

Snowball

Sampling

Systematic

Sampling

Stratified

Sampling

Cluster

Sampling

Other Sampling

Techniques

Simple Random

Sampling

• Convenience-- attempts to obtain a sample of convenient elements. Often, respondents are selected because they happen to be in the right place at the right time.

• Judgmental/Purposive-- is a form of convenience sampling in which the population elements are selected based on the judgment of the researcher.

• Quota —consists of developing control categories, or quotas, of population elements.

• Snowball or Referral—a initial group of respondents is selected then the respondents are asked to identify others by referrals.

Non-Probability Sampling

• Simple Random—a sample is drawn randomly from a list of individuals in a population. It can be done through; a. Lottery Sampling; b. Use of Random Number Tables.

• Systematic—it is taking every nth member of the population, where n is the reciprocal of the percentage of the population which you would like to use as a sample.

Probability Sampling

• Cluster– considered a more practical approach to surveys because it samples by groups or clusters of elements rather than by individual elements.

• Stratified– usually employed when the population is composed of several strata or subgroups.

• Multi-Stage—the population is divided into a number of groups or primary stages from which samples are drawn; these are then divided into groups or secondary stages from which samples are drawn, and so on.

Probability Sampling

Determination of Sample Size

• Sample size may be determined by using:

• Subjective methods (less sophisticated methods)

The rule of thumb approach: eg. 5% of population

Conventional approach: eg. Average of sample sizes of similar other studies;

Cost basis approach: The number that can be studied with the available funds;

• Statistical formulae (more sophisticated methods)

• Confidence interval approach.

Sloven’s Formula in Determining Sample Size

n = N / (1 + Ne²)

Where:

n = number of samplesN = total populatione = error margin (.05 or .01)

or level of confidence

Sloven’s Formula in Determining Sample Size

n = N / (1 + Ne²)

Where:

n = ?N = 1000e = .05 margin of error or

95% confidence level

Sloven’s Formula in Determining Sample Size

n = N / (1 + Ne²)

Where:

n = 1000 / (1 + 1000 * 0.05²)n = 1000 / (1 + .0025)n = 285.71 samplings

End!Thank youLesson 6: Sample and Sampling

Dr. Racidon P. Bernarte