Box 1 Table 1.

Two potential study designs in which either (a) four pumps and four catheters of the same type are tested simultaneously or (b) pump and catheter types are balanced during each 48 hr period of data collection, assuming only four pump-catheter units can be used concurrently and each is tested for 48 hr, three times in succession

DOI: http://dx.doi.org/10.7554/eLife.05519.004

Arrangement	Duration	Bench 1	Bench 2	Bench 3	Bench 4
Suboptimal design with potential for confounding
1	48 hrs × 3	P C	P C	P C	P C
2	48 hrs × 3	P c	P c	P c	P c
3	48 hrs × 3	p C	p C	p C	p C
4	48 hrs × 3	p c	p c	p c	p c
Optimal, balanced design
1	48 hrs × 3	P C	P c	p C	p c
2	48 hrs × 3	P c	P C	p c	p C
3	48 hrs × 3	p c	p C	P c	P C
4	48 hrs × 3	p C	p c	P C	P c

There are four benches of equipment being tested, each with one of each type of pump and one of each type of catheter (P = existing pump; p = new pump, C = existing catheter; c = new catheter).