Statistics

• Probability

  P(A') = probability of A not happening

  P(AuB)= probability of A or B

  P(AnB) = probability of A and B

  P(AnB')= Not in both A and B

  P(AuB') =Not A or B

  P(A|B) = probability of A given B
• Quota Sampling

  Sample reflects a trait until the quota for that trait is met

  +: small sample still representative, no sampling frame, quick, easy,cheap, compare groups in a population

  -:potential bias, groups can be costly or inaccurate, increased scope= more costs, non responses are not recorded
• Opportunity Sampling

  Sampling whoever is available and fits the criteria

  +:easy, cheap

  -: unlikely to be representative, dependant on researcher
• Census

  Observes every member of a population so is very accurate, but time consuming, expensive and can't be done when testing destroys the item
• Sample


  Observations taken from a subset of the population. Cheaper, faster, less data to process, but may not be accurate or large enough to include subgroups of the population
• Simple Random

  +: Bias-free, easy, cheap for small populations, equal chance of selection

  -:needs sampling frame, not suitable for large populations
• Systematic 


  1st person is random then regular intervals

  +:simple, quick, suitable for large populations

  -:needs sampling frame, introduces bias if the sampling frame is not random
• Stratified


  no. in stratum/no. in population x desired sample size

  +: reflects population structures, proportional representation of groups

  -:must make distinct strata, sampling frame, unsuitable for large populations
• Histograms


  Frequency= frequency density x class width

  Frequency polygon = joined the middle of each bar

  Shows continuous grouped data and gives a good idea of distribution
• Comparing Data


  Use the mean and standard deviation OR the median and IQR
• Regression

  +b = positive correlation -b=negative correlation

  using x to find y in the know range = interpolation

  using x to find y outside the range = extrapolation

  using y to x = illegal