statistics year 1 sampling

Population

Whole set of items that are of interest

Raw data

Information obtained from a population

Sample

Selection of observations taken from a subset of a population

Advantages of a census

Results should be completely accurate

Disadvantages of a census

Time consuming, expensive, hard to process a large quantity of data and can’t be used when testing destroys process

Advantages of a sample

Less time consuming, cheap, fewer people have to respond and less data needs to be processed

Disadvantages of sampling

Data may not be as accurate, sample may not be large enough to give information about small sub groups of the population

Random sampling

Every sample has an equal chance of being selected

Types of random sampling

Simple random sampling, systematic sampling and stratified sampling

Random number generator sampling

Number each member and use a random number generator to generate random numbers then select the members that correspond to each number

Advantages of random number sampling

Free of bias, easy and cheap for small populations

Disadvantages for random number sampling

Not suitable for large populations and a sampling frame is needed

Stratified sampling

Find number of sampled in each stratum using the formula then number each member and use a random number generator to produce required quantity if random numbers and select the members that correspond to each number until stratum is filled

Stratified sampling formula

(Number in stratum/ population) X sample size

Advantages of stratified sampling

Sample accurately reflects population structure and no bias

Disadvantages of stratified sampling

Population must be clearly classified in distinct strata, sampling frame needed and not suitable for large populations

Types of non random sampling

Quota and opportunity sampling

Quota sampling

An interview selects a sample that reflects the characteristics of the whole population

Advantages of quota sampling

No sampling frame needed, quick, easy, inexpensive, easy comparison between different groups within a population and allows a small sample to still be representative

Disadvantages of quota sampling

Can be biased as non-random, population has to be divided into groups which can be costly or inaccurate and non-responses not recorded

Opportunity sampling

Sample is taken from people who are available at the time of the study and who fit the criteria

Advantages of opportunity sampling

Quick, easy and inexpensive

Disadvantages of opportunity sampling

Unlikely to produce a representative result and highly dependent on researcher