Directional: States an actual expectation, such as,
Fluoride varnish will prevent root caries more effectively than fluoride gel.
Non-directional: States an expectation but not sure about the direction, e.g., There will be a difference between fluoride varnish and fluoride gel in preventing root caries.
Null: States there is no difference or an absence of a relationship between two variables, e.g., There is no difference between fluoride varnish and fluoride gel in preventing root caries.