If two methods agree perfectly in a method comparison study, the slope equals _____ and the y-intercept equals _____.

• A. 1.0, 0.0
• B. 0.0, 1.0
• C. 1.0, 1.0
• D. 0.0, 0.0

Answer: (A)

When two measurement methods agree perfectly, the relationship between their results follows the identity line, meaning y equals x for all values. In regression terms, that line has a slope of 1 and an intercept of 0, so there is no proportional bias (slope not equal to 1 would imply changing bias with magnitude) and no constant bias (nonzero intercept would shift all points by a fixed amount). Therefore, the correct values are a slope of 1.0 and a y-intercept of 0.0. If the slope were 0, the line would be flat and show no dependence on the x value, which contradicts perfect agreement. If the intercept were nonzero, there would be a constant bias between the methods.