Median blood pressure, as a fuction of salt intake, for each of 52 human populations.
A data frame with 52 observations on the following 4 variables.
a numeric vector
mean diastolic blood pressure (mm Hg)
mean sodium excretion (mmol/24h)
a character vector
For each population took a sample of 25 males and 25 females from each decade in the age range 20 - 50, i.e. 200 individuals in all.
Intersalt Cooperative Research Group. 1988. Intersalt: an international study of electrolyte excretion and blood pressure: results for 24 hour urinary sodium and potassium excretion. British Medical Journal 297: 319-328.
Maindonald, J.H. The Design of Research Studies ? A Statistical Perspective, viii + 109pp. Graduate School Occasional Paper 00/2, Australian National University 2000.
data(intersalt) plot(bp ~ na, data=intersalt, xlab="Median sodium excretion (mmol/24h)", ylab="Median diatoluc blood pressure (mm Hg)")