In the Bivariate platform, the Fit Passing Bablok option uses the Passing-Bablok procedure to fit the linear equation y = β0 + β1x where both x and y are measured with error. The method is robust to outliers and is appropriate if there is a linear relationship between x and y. The estimate of the slope (β1) is calculated as the median of all slopes that can be formed from all possible pairs of data points, except those rare pairs that result in an undefined slope of 0/0 or a slope of -1. To correct for estimation bias caused by the lack of independence of these slopes, the median is shifted by a factor based on K, the number of slopes that are less than -1. The result is an approximately unbiased estimator for β1. The intercept (β0) is estimated by the median of {yi - β1xi}. See Passing and Bablok (1983), Passing and Bablok (1984), and Bablok et. al. (1988).
Passing-Bablok is a commonly used for method comparison studies. The intercept is interpreted as the systematic bias (difference) between the two methods. The slope measures the amount of proportional bias (difference) between the two methods.