A study by Gorecka et al. (A-26) assessed the manner in which among middle-aged smokers the diagnosis of airflow limitation (AL) combined with advice to stop smoking influences the smoking cessation rate. Their concerns were whether having AL, whether the subject successfully quit smoking, and whether interaction between AL and smoking status were significant factors in regard to baseline variables and lung capacity variables at the end of the study. Some of the variables of interest were previous years of smoking (pack years), age at which subject first began smoking, forced expiratory volume in one second ðFEV1Þ, and forced vital capacity (FVC). There were 368 subjects in the study. What are the factors in this study? At how many levels does each occur? Who are the subjects? What is (are) the response variable(s)? Can you think of any extraneous variables whose effects are included in the error term?