##### 311 Unit 7 Study Guide

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1

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
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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
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Monotonic vs. Linear

Monotonic correlation examples

• Memorizing a list of 25 words (number of repetitions required)
• Practicing a skill (like knitting)
• Impact of sleep deprivation
4

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
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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
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Third variable problem and causation

7

Directionality and causation

8

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
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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
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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
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Null hypothesis of correlation

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