Unit 1: Sampling

• What is a population?

  The whole set of items that are of interest
• What is a census?

  It measures every member of a population
• What is a sample?

  A selection of observations taken from a subset of a population to find out more about the population as a whole.
• What are some advantages of a census?

  • It involves the whole population so is a large amount of data
  • It will give very accurate results
• What are some disadvantages of a census?

  • It is time consuming and expensive
  • It cannot be used when the testing process destroys the item
  • Hard to process a large amount of data
• What are some advantages of a sample?

  • Less time consuming and expensive compared to a census
  • Less data to process
  • Less people needed to respond
• What are some disadvantages to a sample?

  • The data may not be as accurate
  • The sample may not be large enough to give information about small sub-groups
• The size of the sample can affect the validity of the conclusions

  1. The size of the sample depends on the required accuracy and available resources
  2. The larger the sample, the more accurate
  3. If the population is varied, then a large sample is needed than when it is uniform
  4. Different samples can lead to different conclusions due to natural variation in a population
• What is the definition of sampling units?

  Individual units of a population
• What is a sampling frame?

  Sampling units that are named and numbered to form a list
• What is a simple random sample?

  Is when everyone in the sample has a chance at being selected
• How do you carry out a simple random sample?

  • You need a sampling frame
  • Everyone should be allocated a unique number
  • To choose numbers randomly you can use a random number generator
• What are the advantages of a simple random sample?

  1. Free of bias
  2. Easy and cheap
  3. Everyone has an equal chance of being selected
• What are the disadvantages of a simple random sample?

  1. Not suitable for a larger population
  2. Sampling frame is needed
• What is systematic sampling?

  When members are chosen at regular intervals from an ordered list
• What are the advantages of systematic sampling?

  1. Simple and quick to use
  2. Suitable for a large sample
• What are the disadvantages of a systematic sample?

  1. Sampling frame is needed
  2. Possible bias if the sampling frame is not random
• What is a stratified sample?

  • Population is divided into groups e.g males and females
  • Random sample is taken from each group so it reflects the population as a whole and removes bias
• What is the equation needed for stratified sampling?

  strata/total * sample size
• What are the disadvantages of a stratified sample?

  1. Population must be classified into groups which is time consuming
  2. Because selection of groups is random, a sampling frame is needed and it is not suitable for a large population
• What are the advantages of a stratified sample?

  1. Sample accurately reflects population
  2. Guarantees proportional representation of groups
• What is opportunity sampling?

  • The sample is taken from people available at the time and who fit the criteria needed
  • e.g speaking with people leaving a supermarket to get their opinions
• What are the advantages with the opportunity sample?

  1. Easy to carry out
  2. Cheap
• What are the disadvantages of the opportunity sample?

  1. Bias
  2. People might not want to be interviewed
  3. Unlikely to give a proportional sample
• What is quota sampling?

  • Researcher selects a sample that reflects the characteristics of the group
  • e.g interviewer might meet with factory members to determine characteristics
• What are the advantages of quota sampling?

  1. Allows a small sample to represent the whole population
  2. No sampling frame is needed
  3. Quick and inexpensive
• What are the disadvantages of quota sampling?

  1. Bias
  2. Some people might not want to take part
• How can you carry out a stratified sample?

  1. To get the first data point you use a random number generator
  2. Then you can choose every nth data point after that