Data Collection Chapter 1

What is a population?

The whole set of items that are of interest

What is information obtained from a population known as?

Raw data

What is a census?

A census measures or observes every member of a population

What is a sample?

Selection of observations taken from a subset of a population and used to find out more information about the population as a whole

What are the advantages of a Census?

Results should be completely accurate

What are the disadvantages of a census?

Time consuming

Expensive

Cannot be used when testing destroys process

What are the advantages of a sample?

Less time consuming and cheaper

Fewer people have to respond

Less data needs to be processed

What are the disadvantages of a sample?

Data may not be as accurate

Sample may not be large enough to give information about small subgroups of the population

What is a sampling unit?

Individual units of a population

What is a sampling frame

When sampling units are numbered to form a list

What is random sampling?

When each member of the population has an equal chance of being selected.

The sample should be representative and bias should be removed

How many different types of random sampling are there?

3

What are the 3 different types of random sampling?

Simple random sampling

Systemic sampling

Stratified sampling

What is simple random sampling?

A simple random sample of size n is one where every sample of size n has an equal chance of being selected

How is simple random sampling carried out?

To carry out a simple random sample, you need a sampling frame, usually a list of people or things.

Each person or thing is allocated a unique number and a selection of these numbers is chosen at random.

There are two methods of choosing the numbers: generating random numbers (using a calculator, computer or random number table) and lottery sampling.

In lottery sampling, the members of the sampling frame could be written on tickets and placed into a ‘hat’. The required number of tickets would then be drawn out.

What are the advantages of simple random sampling?

Free of bias

Easy and cheap for small samples

What are the disadvantages of simple random sampling?

Not suitable for large samples

Sampling frame needed

What is systematic sampling?

The required elements are chosen at regular intervals from an ordered list

How is systemic sampling carried out?

For example, if a sample of size 20 was required from a population of 100, you would take every fifth person since 100 ÷ 20 = 5.

The first person to be chosen should be chosen at random. So, for example, if the first person chosen is number 2 in the list, the remaining sample would be persons 7, 12, 17 etc

What are the advantages of systematic sampling?

Simple and quick to use

Suitable for large populations

What are the disadvantages of systematic sampling?

A sampling frame is needed

Bias is introduced if sampling frame is not random

What is stratified sampling?

The population is divided into mutually exclusive strata and a random sample is taken from each

What is the formula for stratified sampling?

Number sampled in a stratum = Number in a stratum/Number in population x overall sample size

How is stratified sampling carried out?

• Find total number of individuals in population

• Work out number of individuals needed from each strata for the sample

• Number individuals in each group

• Use a random number generator to produce the required quantity of random numbers and match each numbered individual to the random number generated

A factory manager wants to find out about what his workers think about the factory canteen facilities. he decides to give a questionnaire to a sample of 80 workers. It is thought that different age groups will have different opinions.

There are 75 workers between ages 18 and 32, 140 workers between ages 33 and 47, and 85 workers between ages 48 and 62.

Explain how he can use stratified sampling to select the sample

Total number of workers: 75+140+85=300

Finding the number of workers needed from each age group.

20, 37, 23

Number the workers in each group

Use a random number generator or table to produce the required quantity of random numbers

What are the advantages of stratified sampling?

Proportional representation of group within population

What are the disadvantages of stratified sampling?

Population must be clearly classified into distinct strata

Same disadvantages as simple random sampling within each strata

What are the two types of non-random sampling?

Quota sampling

Opportunity sampling

What is quota sampling?

An interviewer or researcher selects a sample that reflects the characteristics of the whole population

What are the advantages of quota sampling?

Allows a small sample to still be representative of the population

No sampling frame required

Quick and easy

What are the disadvantages of quota sampling?

Non-random sampling can introduce bias

Population must be divided into groups, costly or inaccurate

What is opportunity sampling?

A sample taken from people who are available at the time of study and fits the criteria you are looking for

What are the advantages of opportunity sampling?

Easy and inexpensive

What are the disadvantages of opportunity sampling?

Unlikely to provide a representative result

Highly dependent on individual researcher

What is discrete data?

Data that only takes certain values, whole numbers

What is continuous data?

Data that can take any value, eg height weight temperature