A statement of an expectation of no difference is called a null hypothesis.
Null hypothesis is a proposition that undergoes verification to determine if it should be accepted or rejected in favor of an alternative proposition. Often the null hypothesis is expressed as "There is no relationship between two quantities."
In any experimental research study, researchers begin with a question. From this question, researchers derive a null hypothesis - the prediction that a change in the independent variable (the element of the study that is altered in the experimental condition) will have no effect on the dependent variable (the element of the study that is observed for changes upon application of the experimental condition). - Nosignificantdifference.org
Typically, an experimental hypothesis is a statement of what might be expected to occur if the experimental condition in an experiment differed from the control condition, or, in situations where two different treatments are compared, if a difference existed between those two treatments.
Our hypothesis in the preoperative anxiety is an experimental hypothesis. A statement of an expectation of no difference would be stated as follows: 'That there will be no difference in preoperative anxiety bewtwwen patients receiving premedication and those receiving instruction in relaxation training'.
Researchers typically speak of rejecting the null hypothesis and accepting the experimental hypothesis when there is a difference between the experimental groups (in this case, relaxation and premedication), and accepting the null hypothesis when there is not.