The Federal Trade Commission annually rates varieties of domestic
cigarettes according to their tar, nicotine, and carbon monoxide
content.  The United States Surgeon General considers each of these
substances hazardous to a smoker's health.  Past studies have shown
that increases in the tar and nicotine content of a cigarette are
accompanied by an increase in the carbon monoxide emitted from the
cigarette smoke.

The data n this dataset are taken from Mendenhall and Sincich (1992)
and are a subset of the data produced by the Federal Trade Commission.  For a
random sample of 25 brands of cigarettes, 5 variables are reported:

    brand: Brand name
    tar: Tar content (mg)
    nicotine: Nicotine content (mg)
    weight: Weight (g)
    co1:  Carbon monoxide content (mg)