Null & Alternative Hypotheses in Experimental Design: Testing & Decision Rules, Slides of Psychology

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Hypothesis Testing

Experimental Design

• Purpose –
  • Allows a scientist to test the influence of the independent variable upon the dependent variable
  • Controls for the influence of other variables

Experimental Designs

• One Experimental Group
• Two Experimental Groups
  • Experimental – Receives treatment
  • Control Group – No Treatment (placebo)

Hypothesis

• Alternative Hypothesis (H 1 )
  • Results are due to the effect of the independent variable
• Null Hypothesis (H 0 )
  • Results are not due to the effect of the independent variable

Hypothesis

• The alternative hypothesis vs. the null

hypothesis

• If one is true the other must be false and vice versa
• The experiment attempts to show that the null hypothesis is false
• If the null hypothesis is false, the alternative hypothesis can be accepted as true

Decision Rule

• The null hypothesis is evaluated directly

because it is possible to calculate the

probability of chance events

• The focus in on testing the null hypothesis to

support accepting the alternative hypothesis

Decision Errors

• Type I – Null hypothesis is rejected when it is

actually true

• Probability of Type I error set by alpha
• Type II – Null hypothesis is retained when it is

actually false

• Probability of making a Type II error is called beta

Decision Rule

• Level of alpha is dependent upon the particular experiment

Alpha level =

Common levels for alpha

α

α

α

=

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