311 Unit 7 Study Guide

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Correlational research strategy

  • Two or more variables are measured to obtain a set of scores (usually 2) for each individual (or source)
  • Measurements are examined to identify patterns that exist between the variables and to measure the strength of the relationship

What correlations describe

  • The direction of the relation: Positive or negative relations
  • The form of the relation: Linear (using Pearson correlation) or Monotonic (using Spearman correlation)
  • The consistency or strength of relation: How close the relation is to a perfect linear or a perfect monotone relation

Monotonic vs. Linear

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Monotonic correlation examples

  • Memorizing a list of 25 words (number of repetitions required)
  • Practicing a skill (like knitting)
  • Impact of sleep deprivation

Uses of correlational studies

  • Predict: Determine relation between variables, if you know the value of one variable, can you predict the value on another variables?
  • Evaluate: If you have a theory about a relation between variables, correlation can be the first step in assessing the quality/accuracy of that theory
  • Assess validity: For example, to establish convergent and divergent validity, researchers assess relation between score on the new instrument against a score on another instrument
  • Assess reliability: For example, test-retest reliability is the calculation of the correlation between one test score and and another, when you re-take the test a second time

When manipulation is unethical

  • Correlation is useful when variables cannot be ethically manipulated for purposes of an experiment
  • Examples: Do vaccines cause autism? Does smoking cause cancer? How many people die from texting and driving?
  • Also, more ethical early in a line of research, when you don’t know if something might help or hurt

Third variable problem and causation

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Directionality and causation

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Direction of relations

Positive relations

  • Tendency for two variables to change in the same direction
  • As one variable increases, the other also tends to increase

Negative relations

  • Tendency for two variable to change in opposite directions
  • As one variable increases, the other tends to decrease

Correlation coefficient

  • A numerical value that measures or describes the relation between two variables, typically represented by r
  • The direction of relation: Indicated by the correlation’s sign (+ -)
  • The form of relation: Indicated by type (Pearson or Spearman)
  • The consistency or strength of relation:
    • Indicated by correlation numeric value
    • Range = -1.0 to 1.0
    • 0 = no relation

Strength of correlation

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Significance of a correlation

  • A significant correlation means that the correlation is unlikely to have been produced by random variation
  • Small samples (i.e., 2 people) can produce significant correlations when no relationship is actually present
  • As sample size increases, likelihood of significant actual correlations becomes stronger

Null hypothesis of correlation

  • Null hypothesis: r = 0
  • Alternative hypothesis: r ≠ 0

Reporting correlation

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