NAMCS-The survey utilizes a three stage sampling design based on (1) probability of selecting a primary sampling unit (PSU) (2) probability of selecting a physician within the PSU and (3) probability of selecting a patient within the physician practice. This last probability is defined to be the exact number of office visits during physician's specified reporting week divided by the number of patient record forms completed. NHAMCS- For producing unbiased national estimate, this survey utilizes four stages (a) Probability of selecting a PSU (b) probability of selecting a hospital with in PSU (c) probability of selecting an emergency department (ED) or outpatient department (OPD) within the hospital (d) probability of selecting a visit within ED or OPD. The overall probability of selection is the product of the probabilities at each stage. The inverse of the overall selection probability is the basic inflation weight.