Probability and Significance

• What is the definition of probability? [2]

  A measurement of the likelihood that a particular event will occur where 0 indicates statistical impossibilty and 1 indicates statistical certianty.
• What's the definition of significance? [2]

  A statistical term that tells us how sure we are that a difference or correlation exists. A significant result means that the researcher can reject the null hypothesis and accept the alternative hypothesis.
• When looking at descriptive statistics we know that there are some differences but we don't know how significant they are. When a difference is indicated there are two possibilities:

  • The independent variable causes the dependent variable.
  • Any observable difference is due to a random error and NOT the independent variable.
• What is the usual level of significance in Psychology?

  P ≤ 0.05
• What does a probability of P ≤ 0.05 mean? [2]

  It means that the probability that the observed effects occurred by chance is equal to or less than 5%. So there's a 95% chance that the observed effects are due to the effects of the independent variable.
• What is a type I error? [2]

  This is when the researcher says that the results are significant when in fact they are not. They reject the null hypothesis and accept the alternative.
• When is a type I album most likely to happen?

  If the significance level is too lenient (high) at 0.1 (10%)
• What is a type II error? [2]

  When the researcher says that the results are not significant when in fact they are. They accept the null hypothesis and reject the alternative.
• When is a type II error most likely to happen?

  If the significance level is too stringent (low) at 0.01 (1%)
• Why do psychologists prefer the 5% level of significance?

  It best balances the risk of making type I and type II errors.
• What do statistical tests work on the basis of?

  They work on the basis of probability.