Biostatistics, Chapters I & II

Sampling

  • Population: complete collection of all measurements or data that are being considered.
  • Sample: sub-collecion of members selected from a population
  • Simple Random Sample: each member of the population has the same change of being included, and samples are chosen independently
  • Cluster Sampling: dividing the population into groups by a category. All of the individuals within the single group are the sample.
  • Stratified Random Sampling: divide the population into groups (strata) based on one+ classification criteria. Then perform a simple random sample within each strata
  • Sampling Bias: some members of the population have a higher chance to be selected than others.

Variables

  • Categorical Variables: two+ categories, but no intrinsic ordering (ex: blood type)
  • Ordinal Variable: categorical variables but with a clear ordering (small/medium/large)
  • Numeric Variables
      * Discrete Variables: a numeric variable for which we can list the possible values (think: integers)
      * Continuous Variable: a numeric variable that is measured on a continuous scale (temperature, height)
  • Bar Charts: frequency distribution for categorical variables
  • Histograms: frequency distribution but no spaces

Frequency Variables

  • Mean, denoted by ȳ
      * Mean: The average of the observations
      * Only for discrete or continuous data
      * ȳ = (Σ yi)/(n)
      * Sensitive to outliers
  • Median, denoted by ỹ
      * N is odd: (n + 1)th largest value
      * N is even: average of (n/2)th largest value and (n/(2) + 1)th
  • Symmetric and Unimodal Curve
      *
  • Symmetric and Multimodal Curve
      *

Box Plots

  • Quartiles
      * Q1 = 25th Percentile
      * Q2 = 50th Percentile (Median)
      * Q3 = 75th Percentile
  • Fences
      * LF = Q1 - h
      * UF = Q3 + h
      * h = 1.5(Q3 - Q1)
      * Outliers are any points that lie outside of the LF and UF
  • Drawing a Box Plot
      * Central box from Q1 to Q3
      * Line in the middle is Q2
      * Whiskers extend to the point CLOSEST to the LF & UF (not the actual values of the fences)
      * Outliers are marked by small circles

Label y axis

Variance

  • Sample variance
      * s^2 = Σ(yi - ȳ)^2 / n - 1
      * Remember to subtract one from n
  • Simple Standard deviation
      * Sqrt(s^2)
      * Same unit as the original data value
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