Sampling, data presentation and interpretation

• Advantage of quota sampling

  Quick and easy No sampling frame needed Allows for easy comparison Allows a small sample to still be represented of the population
• Advantage of opportunity sampling

  Easy to carry out
• Advantage of a sample

  Quicker Less expensive Easier to process
• Advantage of a census

  Representative So accurate result
• Advantage of stratified sampling

  Accurately reflects the population structure Representative
• Advantage of systematic samples

  Suitable for large samples Quick and easy to use
• Advantages of simple random sample

  Free of bias Easy and cheap
• Disadvantage of census

  Time consuming Expensive Cannot be used when the testing process destroys the item
• Disadvantage of opportunity sampling

  Unlikely to be Representative
• Disadvantage of quota sampling

  Can be bias Non responses are ignored
• Disadvantage of quota sampling

  Can be bias Non responses are ignored
• Disadvantage of samples

  Data may not be accurate or representative
• Disadvantage of stratified sampling

  Population must be clearly classified into strata Not suitable for large Population or sample size
• Disadvantage of systematic samples

  Sampling frame is needed Can be bias if sampling frame is not random
• Disadvantage of systematic samples

  Sampling frame is needed Can be bias if sampling frame is not random
• Disadvantages of simple random samples

  Sampling frame is needed Not suitable when population or sample size is large
• Opportunity sampling

  Taken from people who are available at the time Eg the first 20 people to walk into a shop
• Quota sampling

  Researcher selects a sample that reflects the characteristics of the whole population Population divided into groups according to given characteristics
• Sampling frame

  A list of the population
• Sampling units

  Individual items of a population Eg a resident
• Simple random sample

  Give each member if the population a unique number Use a random number generator to pick out the numbers ignoring repeats
• Stratified sampling

  Population is divided into categories Random sample taken from each of them The sample is reflective of the population Eg if there are 60% females 60% of the sample will be female
• Stratified sampling

  Population is divided into categories Random sample taken from each of them The sample is reflective of the population Eg if there are 60% females 60% of the sample will be female
• Stratified sampling equation

  Number in stratum/number in population x sample size
• Systematic sample

  The required elements are chosen at regular intervals
• Systematic sample

  The required elements are chosen at regular intervals
• Test statistic

  Results from the experiment Eg the number of heads obtained
• Types of non random sampling

  Quota Opportunity
• Types of non random sampling

  Quota Opportunity
• Types of random sample

  Simple random Systematic Stratified
• What is a census

  Where data from every member of the population is obtained
• What is a random sample

  Where every member of the population has an equal chance of being selected
• What is a sample

  Where data from a subset of the population is obtained
• Formula for coded values


  y = x - a /b
• Formula for coded mean

  Coded mean = mean - a /b
• Formula for coded standard deviation

  Coded standard deviation = standard deviation - a /b
• How to use interpolation to find a measure of location eg the median

  Lcb = lower class boundary

  Ucb =upper class boundary

  ucv = upper cumulative value

  lcv - lower cumulative value

  median - half of the total frequency

  Q2 - lcb / ucb - lcb = median -lcv/ ucv - lcv

  
• What do you use to find a measure of location in continous data

  Interpolation - a number line
• How to calculate lower quartile (Q1) for discrete data

  If 1/4 n is a whole number Q1 is halfway between this data point and the next

  If 1/4 n is not a whole number round up and pick this data point
• How to calculate median (Q2) for discrete data

  If 1/2 n is a whole number Q2 is halfway between this data point and the next If 1/2 n is not a whole number round up and pick this data point