Preferred provider organization
In health insurance in the United States, a preferred provider organization (PPO), sometimes referred to as a participating provider organization or preferred provider option, is a managed care organization of medical doctors, hospitals, and other health care providers who have agreed with an insurer or a third-party administrator to provide health care at reduced rates to the insurer's or administrator's clients.
Predictive probability of success
Traditional pilot trial design is typically done by controlling type I error rate and power for detecting a specific parameter value. The goal of a pilot trials such as a phase II trial is usually not to support registration. Therefore, it doesn't make sense to control type I error rate especially a big type I error as typically done in a phase II trial. A pilot trail usually provides evidence to support a Go/No Go decision for a confirmatory trial. Therefore, it makes more sense to design a trial based on PPOS. 
To support a No/Go decision, traditional methods require the PPOS to be small. However the PPOS can be small just due to chance. To solve this issue, we can require the PPOS credible interval to be tight such that the PPOS calculation is supported by sufficient information and hence PPOS is not small just due to chance. 
Finding an optimal design is equivalent to find the solution to the following 2 equations.
where PPOS1 and PPOS2 are some user-defined cutoff values. The first equation ensures that the PPOS is small such that not too many trials will be prevented entering next stage to guard against false negative. The first equation also ensures that the PPOS is not too small such that not too many trials will enter the next stage to guard against false positive. The second equation ensures that the PPOS credible interval is tight such that the PPOS calculation is supported by sufficient information. The second equation also ensures that the PPOS credible interval is not too tight such that it won't demand too much resource.