Types of statistical calculations for sample size

Different formula for each type of test.

Mathematically derived from formulae used for hypothesis tests.

High chance of false positive (large О±).

Precise outcome measure (small standard deviation).

There will be a smaller sample size with: Higher power is better, aim for at least 90% power. the chance of spotting a difference as being statistically significant if there really is a difference. The chance of not getting a false negative result. the chance of not detecting a significant difference when there really is a difference. the chance of detecting a statistically significant difference when there is no real difference between treatments. Definitions Possible results from a clinical study Sample size calculations are sometimes referred to as power calculations, for reasons that should be obvious when you have finished reading this document.

it is unethical to inconvenience patients if a study could not have detected a useful effect.journals and research funding bodies require sample sizes calculations aВ priori.'negative results' can be useful if a real difference could have been detected.if the sample size is too small a real difference between treatments could be missed.Why bother with sample size calculations? Tables and equations for most situations, lots of examples. Sample Size Tables for Clinical Studies, 2ndВ ed. Written for medics, contains a brief summary, equations and examples. Medical Statistics, A Commonsense Approach, 2ndВ ed., pp.В 156-157. Similar to, but slightly more mathematical than Altman's book.Д¬ampbellВ M.J. Written for medics, gives simple formulae and basic tables. An Introduction to Medical Statistics, 2ndВ ed., pp.В 331-341. Written for medics, contains a good introduction to sample size calculationsД«landВ M., 1995. Practical statistics for medical research, pp.В 455-460. (Also appears in: D.G.В Altman & S.M.В Gore, 1982. Written for medics, good brief introduction to sample size calculations. Describe the relationships between sample size, power, significance level and standard deviationДЄltman,В D.G., Statistics and ethics in medical research: III How large a sample? BMJВ 281:В 1336-8.Explain the concepts of TypeВ 1 errors, TypeВ 2 errors and statistical power.Outline the necessity of aВ priori sample size calculations.At the end of the lecture and having completed the exercises students should be able to:

Types of statistical calculations for sample size