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StatsToDo : Sample Size for Phase II Study (Fleming's Procedure)
Explained, Calculations, and Table

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Phase II Studies Explained Page

Introduction Calculations Sample Size Table
This page provides the table for sample size required for a Phase II study

To design a Phase II trial of a new drug or treatment, the existing probability of success (survival or cure) (p0), and the improved proportion of success with the new treatment being studied (pn) are defined. The Type I Error (usually 0.05 or 0.1, and the power (usually 0.8 or 0.9) are also defined. From these the sample size of the study can be calculated. After all the cases have been treated, the success rate can then be estimated.


We are treating the advanced stage of a particularly aggressive type of cancer, and after the current standard surgery and radiotherapy, only 20% (p0=0.2) of the patients survive 12 or more months. We are offered to trial a newly developed chemotherapy, and we decided that a formal large scale Phase III trial would be worth while if the Phase II trial find the 12 months survival rate is doubled (pn = 0.4).

We use the statistical parameters α=0.05, power=0.8, p0=0.2, pn=0.4, and determined that the sample size is 29 cases.

We administered the additional chemotherapy to 29 patients that fits the description, and 14 (0.48 or 48%) survived at the end of 12 months. As we define success as survival for more than 12 months, We concluded that the results are sufficiently encouraging for a formal phase III trial to be conducted.

Had the 12 month survival rate be less than 12 out of our 29 cases (<0.4 or 40%), then we would have abandoned this new treatment and concluded that further study of it is unlikely to be productive.

Resources on this page

The same calculation is provided in two modes

  • A Javascript web based program allows uswers to estimate the sample size required immediately, without the need to acticate any other applications
  • The calculations, in R codes are presented, allowing the user to copy and incorporate the codes in other applications
For the user's convenience, the sample size required under a variety of common research scenario are also tabled.


Machin D, Campbell M, Fayers, P, Pinol A (1997) Sample Size Tables for Clinical Studies. Second Ed. Blackwell Science IBSN 0-86542-870-0 p. 254-255