Probability and significance

• The vast majority of research you study in Psychology has been accepted due to a test of how likely the data that supports it came about due to chance
• Psychologists look over the data they collect in their research

  Make a decision: Is this data strong enough to accept my alternate hypothesis or do I need to reject my alternate hypothesis and accept the null hypothesis?
• Null hypothesis (H0)


  Hypothesis that suggests there is no causal relationship between the independent and dependent variable
• Alternate hypothesis (H1)


  Hypothesis that suggests there is a causal relationship between the independent and dependent variable
• Experiment


  1. Independent variable is manipulated by artificially creating separate conditions where the only thing that's changed or varied is the independent variable

  2. Resulting change in the dependent variable is carefully measured
• The data collected may not be strong enough evidence to accept the alternate hypothesis
• Probability


  The tool psychologists use to manage the risk of accepting a hypothesis that isn't true or rejecting a hypothesis that is true
• Psychologists accept an alternate hypothesis at the 0.05 level of significance (1 in 20 chance that the results are due to chance)
• Type 1 error


  Accepting an alternate hypothesis as true when in reality it is not true (the positive results were entirely due to chance)
• Type 2 error


  Rejecting a true alternate hypothesis because the data isn't strong enough and too many participants didn't behave as expected just due to chance
• Reducing the likelihood of a Type 1 error by using a stronger level of significance (e.g. 0.01) increases the likelihood of a Type 2 error
• Null hypothesis in psychology

  Similar to the jury's starting perspective that the defendant is innocent until proven guilty
• Alternate hypothesis in psychology


  Similar to the prosecution's argument that the defendant is guilty
• The jury can't know the objective truth, they can only make a decision based on the evidence provided
• Accepting a false alternate hypothesis is a Type 1 error, rejecting a true alternate hypothesis is a Type 2 error
• Probability and significance is a difficult concept for students to understand in psychology