PROBABILITY AND SIGNIFICANCE

• what is probability?

  estimating or calculating how likely or 'probable' something is to happen
• what does a null hypothesis state?

  there is no difference or no correlation between the conditions
• what happens if the statistical test is not significant?

  the null hypothesis is accepted
• what does the statistical test determine?

  which hypothesis (null or alternative) is 'true' and thus which we accept and reject
• what is the significance level?

  the point at which the null hypothesis is accepted or rejected
• what is a P value?

  the probability that the result was obtained by chance and it not significant

  it is the result of a statistical test
• what must psychologists do before a statistical test?

  have to decide on the point at which the probability of the results being due to chance is so small that they can safely say the results must be significant
• what is the usual level of significance?

  0.05 or 5%

  this means the probability that the observed effect occurred by chance is equal to or less than 5%
• what is a type 1 error?

  the null hypothesis is rejected and alternative is accepted when the null hypothesis is 'true'

  this is a optimistic error or a false positive as a significant difference or correlation is found when one does not exist
• what is a type 2 error?

  the null hypothesis is accepted but, in reality, the alternative hypothesis is trye

  this is a pessimistic error or a false negative
• what makes a type 1 error more likely?

  if the significance level is too lenient eg 10%
• what makes a type 2 error more likely?

  if the significance level is too stringent eg 1% as potentially significant values may be missed
• what are the 3 criteria to find the correct critical value?

  hypothesis one tailed or two tailed

  number of participants (N) or degrees of freedom (df)

  level of significance (p value)
• what is compared when checking for statistical significance?

  the calculated value (result of stat test) compared with critical value in a table of critical values based on probabilities