1) Determine what you expect the outcome percentages will be in the two groups: boxes (a) and (b)

For example a trial is being designed to compare two treatments with an outcome of Response to Treatment. It is expected that the test group will show a response rate of 65% and the comparator group a response rate of 40%. Therefore in box (a) you would put 65 and in box (b) you would put 40

2. Determine the significance level: box (c)

This is usually set at 5% so box (c) would be 5. In simple terms this is the acceptable chance of the trial showing a false positive, in other words demonstrating a difference between the treatments when there isn't really one there. The value set here is what the p-value must be less than at the end of the trial to declare a significant difference. If in doubt just leave it at 5% and think no more of it!

3. Determine the Power Level: box (d)

In simple terms power is the chance of the trial demonstrating a significant difference if the assumed values in box (a) and (b) are correct. If the assumed values (a) and (b) are incorrect then the power will not be what is stated. Most trials are conducted with 80% or 90% power. For 80% power put 80 in box (d)

4. Determine the Withdrawal/Non-evaluable Rate: box (e).

The sample size calculator will initially determine how many subjects are required to complete the trial (and appear in the final analysis). A percentage of patients will usually not complete the trial and therefore will not contribute to the final analysis. If you expect that 10% of your subjects may do this then put 10 in box (e)