(Y1) 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.