Statistical planning and Sample size determination.

Statistical planning Statistical planning and Sample size and Sample size determinationdetermination

Statistical planning Statistical planning and Sample size and Sample size determinationdetermination

Planning• Which variables will I collect and how

will they be measured?• What types of data do they represent?• What do I want to do? Describe,

compare groups, predict change in Y following change in X?

• How large a sample do I need to find a clinically important difference statistically significant???

Dummy analysis• How will I describe my data?

• What hypothesis tests will I be doing?

Sample size

Descriptive study• Here the sample size will

determine the precision of the study (the confidence interval)

• Small study>>>> wide CI• Large study>>>>narrow CI• Here power and P values are not

relevant

Mean 120 mmHg Sd 20 mmHg What sample size needed for CI to be ± 2.2

Hypothesis testing• Studies where we test hypothesis

eg compare groups regarding means or proportions or test significance of correlation or regression coefficients

Type 1 and 2 error

Method 1• Working with standardised

difference and nomogram

Method 2Working with 2 means and Sd or

working with two proportions

N=130

One tailed table (must X 2)

• Use same nomogram

N=3000 in total

Examples