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