statistical testing

• Significance:

  The difference/association between two sets of data is greater than what would occur by chance- coincidence or fluke. To find out if the difference/association is significant we need to use a statistical test.
• Probability:


  Probability (p) is a numerical measure of the likelihood that certain events will occur. The accepted level of probability in psychology is 0.05 (a significance level of 5%). This is the level at which the researcher decides to accept the alternative hypothesis or not. If the alternative hypothesis is accepted, there is less than 5% chance that the null hypothesis is true, i.e. a probability of 0.05 that there was no real effect in the population from which the sample was drawn.
• What is the accepted level of probability in psychology?

  0.05 (a significance level of 5%).
• Calculated value:

  The calculated value is compared with a critical value to decide whether the result is significant or not.
• Critical value:

  The critical values for a particular test are given in a table of critical values.
• Finding the critical value:

  To find the critical value you need to know:                                                  1) The significance level (usually 0.05 or 5%)                                                2) The number of participants in the investigation (the N value) or the degrees of freedom (df).                                                                            3)Whether the hypothesis is directional or non-directional.
• Conditions of use for the sign test:

  used to analyse the difference in scores between related items, e.g. the same participant is tested twice. Can be used with nominal data.
• Calculation in the sign test:

  • The score for condition B is subtracted from condition A to produce the sign of difference (either a plus or a minus)
  • The total number of pluses and minuses should be calculated.
  • Participants who achieved the same score in condition A and condition B should be disregarded, and deducted from the N value.
  • The S value is the total of the less frequent sign.
• Critical value in the sign test:

  If S is equal to or less than the critical value, then S is significant and the experimental hypothesis is retained.