Sampling

Census

Observes or measures every member of a population

Advantages of a census

Completely accurate

Disadvantages of census

Time consuming and expensive

Cannot be used if the testing process destroys them

Hard to process a large quantity of data

Sample

A selection of observations taken from a subset of the population which is used to find out information about the population as a whole

Simple random sampling

Every sample of size n has a known and equal chance of selection

Advantages of simple random sampling

Free of bias

Easy and cheap to implement for small populations and small samples

Each sampling unit has a known and equal chance of selection

Disadvantages of simple random sampling

Not suitable when the population size or the sample size is large

A sampling frame is needed

Systematic sampling

Elements are chosen at regular intervals from an ordered list

Advantages of Systematic sampling

Simple and quick to use

Suitable for large samples and large populations

Disadvantages of systematic sampling

A sampling frame is needed

It can introduce bias is the sampling frame is not random

Stratified sampling

Population is divided up into mutually exclusive strata (e.g. men and woman) and a random sample is taken from each

Advantages of stratified sampling

sample accurately reflects the population structure

Guarantees proportional representation of groups within a population

Disadvantages of stratified sampling

Population must be clearly classified into distinct strata

Selection within each stratum suffers from the same disadvantages as simple random sampling

Quota sampling

Researcher selects a sample that reflects the characteristics of the population

Advantages of Quota sampling

Allows a small sample to still be representative of the population

No sampling frame required

Quick, easy and inexpensive

Allows for easy comparison between different groups within a population

Disadvantages of Quota sampling

Non-random sampling can introduce bias

Population must be divided into groups, which can be time consuming and expensive

Increasing scope of study increases number of groups, which adds time and money

Non-responses are recorded as such

Opportunity sampling

Taking a sample from the people available at the time of the study who fit the criteria you are looking for

Advantages of opportunity sampling

Easy to carry out

Inexpensive

Disadvantages of opportunity sampling

Unlikely to provide a representative sample

Highly dependent on individual researcher

Quantitative variable

Variables with numerical observations

Quantitive data

Data with numerical observations

Qualitive variables

Variables with non-numerical observations

Continuous variable

Can take any value within a given range

Discrete variable

Can take only specific values within a given range

Why should statistical models be used?

To simplify a real world problem

Quicker and cheaper than a census