We begin our analysis by assuming that the Ho is true . This is analogous to the presumption of innocence if you are committed for trial. If the observed difference is less than this chance amount we have insufficient evidence for an effect since the difference could have arisen by chance. In such circumstances we must conclude that there is no significant difference between the two mean values. Again this analogous to finding someone innocence if there was insufficient evidence of guilt.
Many people find the concept of statistical significance rather confusing.
Another difficulty is that statistical significance does not automatically imply practical significance. Although this is a real drop in blood pressure it is of no practical value. It is not possible to define an absolute value for the effect of sampling variation but we can define an effect size that has a specified probability of occurring. The specified probability is called the level of significance and is symbolised by alpha, . We need to assign a value to alpha before we do any analysis otherwise we could still be subjective.