This is a true example analyzing systolic blood pressure in several countries around the world.

Since individual blood pressure presents certain degree of clustering within the countries, these country differences and their confidence intervals are appropriately calculated in multilevel regression analysis.

For more information consult:

Leyland AH, Goldstein H. Multilevel modelling of health statistics. New York: Wiley, 2001.

Snijders TAB, Bosker RJ. Multilevel analysis-an introduction to basic and advanced multilevel modeling. Thousand Oaks, California: SAGE Publications, 1999.