Data collection

Population

The whole set of items

Census

Measures every member of a population

Sample

A selection of observation taken from subset of population, which is representative of population.

Sampling units

Individual units of population

Sampling frame

sampling units of a population are individually numbered to form a list.

Census Advantages and disadvantages

Advantages

• gives completely accurate results

Disadvantages

• Time consuming and expensive

• testing to destruction

• hard to process large quantity of data

Sample advantages and disadvantages

Advantages:

• Less time consuming and expensive than a census

• Less data processed

Disadvantages:

• Sample may not be large enough to represent small sub-groups of population

What is simple random sampling, advantages and disadvantages:

Simple random sampling is where every sampling unit has an equal chance of being selected.

Advantages

• Free of bias

• Cheap to implement on small populations and samples

• Each sampling unit has equal chance of selection

Disadvantages:

• Not suitable when population size is too large

• A sampling frame is needed

What is systematic sampling, advantages and disadvantages?

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

Advantages:

• Simple and quick to use

• Suitable for large samples and populations

Disadvantages:

• A sampling frame is needed

• It can introduce bias if sampling frame is not random

What is stratified sampling, advantages and disadvantages?

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

Advantages:

• Sample reflects the population

• Represents individual groups within the population

Disadvantages:

• sometimes population is not clearly classified into distinct strata

• A sampling frame required

What is Quota sampling, advantages and disadvantages?

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

Advantages:

• Small sample still representative of population

• No sampling frame required

• Quick, easy and inexpensive

Disadvantages:

• Non-random can introduce bias

• Population must be divided into groups which can be inaccurate or costly

What is Opportunity sampling, advantages and disadvantages?

Sample is taken from people who are available at the time the study is carried out.

Advantages:

• Easy to carry out

• Inexpensive

Disadvantages

• Unlikely to provide a representative sample

• Introduces bias

Different types of variables:

• Quantitative: Numerical

• Qualitative: non-numerical

• Continuous: value given a range

• Discrete: only specific values (whole numbers)

Large data set