LECTURE 6

Sampling and Sampling Procedures

GENERAL CONSIDERATIONS

In most epidemiologic studies, we deal with a sample of the population

The study population may be: An entire population A group of the population with certain

characteristics - age - sex - religion - social status Hospital inpatients

THE SAMPLE A sample is a portion of the population,

used for survey purposes when you can not examine the entire population because the resources are not adequate.

A sample should be a representative group of the whole population to be studied.

When a sample is representative to the whole population, generalization from the sample can be made.

A number of techniques can make the sample a representative group of the whole population.

Only the simpler techniques will be considered in this lecture, and when used with care will meet the requirements of epidemiologic studies.

Complex sampling methods need the help of a statistician.

SAMPLE SIZE

The precision with which the investigator make generalization from a sample to the entire population is related to the size of the sample.

THE SAMPLE

Sample selection needs a list of all members (units) of the community from which a sample is selected.

The sampling units may be :1. Individuals2. Families3. Blocks in a city4. Class in a school5. Whole school

SAMPLING PROCEDURES

Probability sampling: which is the most recommended method

because all sampling units have the same probability of being selected and included in the study.

This method assures that the sample is a representative of the whole population.

Inferences from the sample can be made about the whole population (generalization of results to the whole population).

SAMPLING PROCEDURES

Sampling techniques include the following :

Haphazard sample: Ex- in an experiment, where the

experimental animal is chosen as the investigator can catch the animal from the cage.

SAMPLING PROCEDURES

Selected sample: The individuals are selected according to the

investigator’ opinion that they are typical of the population being studied and is appropriate for the study objective .

The sample may be selected because it is handy sample.

selection of groups known to be co-operative The disadvantage of this sample is that you can not

be confident that this sample represents the population, so generalization cannot be made out of this sample.

SAMPLING PROCEDURES

Self-selected sample: The sample consists of individuals who

volunteered for an experiment. Such method can provide some

information about the population and can serve as a basis for future adequate studies

Generalization cannot be made out of this sample.

SIMPLE RANDOM SAMPLE

A sample may be defined as random if every individual in the population has an equal chance of being included in the sample.

Random selection is the basis of this sampling technique.

Any method of selection based on volunteering or selection of groups known to be co-operative is excluded and cannot be used for generalization to the entire population.

Selecting a random sample

It is necessary to have a complete list of sampling

units of the total population: Individual persons House holds Schools cities

Selecting a probability sample

To select a simple random sample: Make a numbered list of the units in

the population (assign a number to each individual)

Estimate the size of the sample Select the required number of sampling

units using a table of random numbers.

Selecting a random sample

If the size of the population is not too large:

Write the numbers on small cards Place them in a bowl and mix them

thoroughly The number representing the sample

size is selected from the bowl.

Selecting a probability sample

Selecting a random sample

Systematic sample ( more efficient): In this sampling method, every nth person

in the list is chosen and included in the sample

If the list contains 10,000 units and the researchers wants 1,000 units, he will select one person every 10 persons (nth =10)

The first person will be selected at random between numbers from 1-10, then every tenth unit will be selected.

CLUSTER SAMPLE:

This sampling method is used when a large survey is planned to cover a district, governorate or the entire country (two stage sampling).

If there is no satisfactory list of the population is available ( there is no list of a city’s population),

a list of groups or clusters of individuals is available, such as city districts, census neighborhoods, villages or schools.

a random sample of these clusters is selected and all individuals in the cluster are included in the study (one stage sampling).

CLUSTER SAMPLE:

An initial sample is taken from the units: Neighborhoods entire district Villages entire governorate Governorates entire country Then a listing of individuals within the

chosen units (clusters) is made and a random sample is taken from each (two stage sampling).

The disadvantage to this method is that disease may be accumulated in some clusters giving misleading results.

Selecting a probability sample

Stratified random sample: If the severity of the disease differs

according one or more characteristics such as sex, age, or socio-economic status, then the population to be sampled should be divided into subgroups or strata according to the characteristics that affect the disease.

Then random sample is selected within each stratum and all individuals in each stratum are included in the study .

Selecting a random sample

Stratified random sample:To assess the prevalence of a disease like carieswhich is unevenly distributed among age groups,we have to obtain a sample stratified by age: 1. The study population is subdivided into age

groups such as 6-12, 13-18, 19-25 and 26 and over.

2. A certain number of each age group is then selected randomly and all individuals in each age stratum are included in the study .

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