Final answer:
The null value for relative risk, which indicates no difference in risk between the two groups, is 1. This value signifies that the incidence of the health-related event is the same in both the exposed and unexposed groups.
Step-by-step explanation:
Relative risk is a ratio, and therefore the null value (no difference) is 1. This is because when calculating relative risk, you divide the incidence of the health event for the exposed group by the incidence of the health event in the unexposed group. If both groups have the same incidence, the ratio is 1, which indicates that there is no difference in risk, as the incidences are approximately equal in both groups. Conversely, ratios greater than or less than one suggest a higher or lower risk, respectively.
To test the null hypothesis in epidemiological studies, which conventionally states that there is no difference (the difference equals 0), relative risk helps determine if there is an association between exposure to a risk factor and a health outcome. The conclusion, when the relative risk equals 1, supports the null hypothesis, indicating no association.