Altman, DG. Practical Statistics for Medical Research London, UK; Chapman & Hall; 1991.
Altman, DG. How large a sample? In: Gore SM, Altman DG. , editor.
Statistics in Practice. London, UK: British Medical Association; 1982.
Whitley E and Ball J (2002) Statistics review 4: Sample size calculations
Crit Care. 6(4): 335-41.
Gerald van Belle (2002). Statistical Rules of Thumb. John Wiley and Sons,
New York. ISBN 0-471-40227-3. p. 45-46