Background To describe a method to determine the effect size of an observed factor for a disease by evaluating the difference in the frequency of disease occurrence between exposed and unexposed groups.
Methods A model of multiple pathogenic factors was established by analyzing the number and distribution of observed factors in a study population. The difference in the incidence between two groups (exposed and unexposed) was calculated according to the model.
Results The distribution of observed factors in the population did not influence the difference in disease incidence between the two groups. However, the difference in incidence between the two groups was able to correctly reflect the number of factors combined in the models, and therefore, indicates that group differences in incidence is a reasonable indicator of effect size. Difference scores <0.25 indicate that one of four or more factors plays a role in a disease; scores >0.50 indicate one of two factors plays a role in disease and implies a high intensity level of the factor.
Conclusions A difference in incidence between two groups over 0.25 is suggested as an indicator of a substantial effect size.