## Dropouts continued [Two-Stage / GS Designs]

Hi ElMaestro,

had a closer look into the loss of power due to dropouts.

» Let us say we look at e.g. CV=.3 and N1=12 (6 per sequence at stage 1).

»

» […] stage 2's are often (not always, just often in practice) much larger than stage 1's…

Larger is OK but

My current practice is to use n

Even with a futility (N

I don’t like small first stages.

had a closer look into the loss of power due to dropouts.

» Let us say we look at e.g. CV=.3 and N1=12 (6 per sequence at stage 1).

**What would you expect in terms of power […], if 2 subjects go lost in sequence RT during stage 2?**»

» […] stage 2's are often (not always, just often in practice) much larger than stage 1's…

Larger is OK but

*too*large isn’t. With the exception of Xu’s methods (my current favorite) you don’t reach the target power with small n_{1}. ‘Potvin B’:` power`

n1 N (median) stage 1 final

12 44 0.0671 0.784

14 44 0.104 0.799

16 44 0.155 0.810

24 36 0.413 0.830

30 30 0.566 0.834

` N power`

44 0.779

43 0.768

42 0.757

41 0.745

40 0.733

_{1}is small you are punished twice: You don’t reach the target power and the total sample sizes will be larger.My current practice is to use n

_{1}which is ~75–80% of the fixed sample design with a ‘best-guess’ CV. With n_{1}30 we exceed the target power with 83.4%, only 41.8% studies proceed to the second stage (*i.e.*, you have a 56.6% chance to show BE already in the first), and the median total sample size is 30 (!). OK, the mean is 39… The distribution looks like that:Even with a futility (N

_{max}64) final power is with 77.6% close to the target but power in the first stage is still 56.6%. With a futility on the total sample size you can not only ease the sponsor but – hopefully – the BSWP. The Type I Error drops from 0.0440 to 0.0427.I don’t like small first stages.

—

Cheers,

Helmut Schütz

The quality of responses received is directly proportional to the quality of the question asked. 🚮

Science Quotes

Cheers,

Helmut Schütz

The quality of responses received is directly proportional to the quality of the question asked. 🚮

Science Quotes

### Complete thread:

- Potvin designs w. imbalance - what would you expect? ElMaestro 2017-08-17 13:02
- Dropouts in stage 2 Helmut 2017-09-12 01:41
- Dropouts continuedHelmut 2017-09-19 17:17