The following is an excerpt from a peer-reviewed journal article: "This study examine(d) the relationship among smoking status and low-income status after accounting for race, number of children in the household, and marital status within one county in the State of Iowa during 2000... "All measurements and covariates were obtained from the State of Iowa licensing information survey. Smoking status was categorized as either being a current or former smoker (smoker = yes), or as being a lifetime non-smoker (smoker = no). Similarly, low-income was determined as being at or below 150% of the federal poverty line (low income = yes) or as having an income greater than 150% of the same standard (low income = no)." Name the study design that was used for this analysis