1.2. Corrected Pearson's chi-square tests. The corrected chi-square approach is based on the idea that the coefficients of the logistic regression of the two viruses can be used to estimate the number of seronegative, single-and double-seropositive individuals expected if the two pathogens are independent. As the classical chi-square, the corrected chi-square compares the observed (O i,j ) and theoretical (E i,j ) numbers of individuals with different combinations of status (seropositive or seronegative) for the two pathogens using the chi-square statistic: