The tb.dilute data frame has 18 rows and 3 columns. It contains
data from a drug test involving dilutions of tuberculin.
tb.diluteThis data frame contains the following columns:
reactiona numeric vector, reaction sizes (average of diameters) for tuberculin skin pricks.
animala factor with levels 1--6.
logdosea factor with levels 0.5, 0, and -0.5.
The actual dilutions were 1:100, \(1:100\sqrt{10}\), 1:1000.
Setting the middle one to 1 and using base-10 logarithms gives
the logdose values.