Format
A data frame with 30 observations on the following 3 variables.
therapy- A factor with 2 levels giving the types of therapy.
drug- A factor with 3 levels giving the drugs administered.
level- A numeric vector giving the patients' hemoglobin levels.
Source
Nelson, P. R., Wludyka, P. S., Copeland, K. A. F. (2005) The Analysis of Means: A Graphical Method for Comparing Means, Rates, and Proportions. Society for Industrial and Applied Mathematics (SIAM), Philadelphia, PA, and American Statistical Association (ASA), Alexandria, VA, pp. 71 ff.