Sampling

• A census measures or observes every member of a population.
• A sample is a selection of observations taken from a subset of a population used to represent the whole population.
• Census advantages include: results should be completely accurate.
• Census disadvantages include: time consuming, expensive, cannot be used when testing destroys the sample, hard to process large quantities of data.
• Sample advantages include: quicker, cheaper, less data needs to be processed, don't need all responses.
• Sample disadvantages include: may not be accurate, may not be representative.
• Sampling frames are a numbered list of all units in a population that are selected to be included in the sample.
• Random sampling is a method of selecting a sample from a population where each unit has an equal chance of being selected. There are three types.
• Simple random sampling involves randomly choosing units without replacement until the desired sample size is reached.
• Simple random sampling advantages include: free of bias, easy and cheap for small samples.
• Simple random sample disadvantages include: sampling frame required, not suitable for large samples.
• Systematic sampling involves choosing units at regular intervals from an ordered list.
• Systematic sampling advantages include: easy and quick to use, suitable for large samples, and can be used to estimate population proportions.
• Systematic sampling disadvantages include: a sampling frame is required, can be biased if sampling frame is not random.
• Stratified sampling involves selecting a sample from a population by dividing it into mutually exclusive subgroups (strata) and then selecting a random sample from each subgroup.
• Number sampled in a stratum equals the number in stratum divided by the number in population multiplied by the overall sample size.
• Stratified sampling advantages include: reflects population structure, proportional representation.
• Stratified sampling disadvantages include: population must be clearly classified into distinct strata, sampling frame required, time consuming.
• Non random sampling is a method of selecting a sample where each unit does not have an equal chance of being selected. There are two types.
• Quota sampling involves selecting a sample from a population based on a set of criteria.
• Quota sampling advantages include: can be representative, no sampling frame required, quick, easy, cheap, easy to analyse.
• Quota sampling disadvantages include: biased, non responses not recorded, increasing accuracy increases time and cost.
• Opportunity sampling involves selecting units available at the time of study.
• Opportunity sampling advantages: easy, quick, cheap, convenient.
• Opportunity sampling disadvantages: unlikely to be representative, biased.
• Quantitative data is data associated with numerical observations.
• Qualitative data is data associated with non numerical observations. It is descriptive and subjective.