Similar to the well known KISS principle ("Keep It Simple Stupid"), the Most-Likely-to-Happen factor is a way for engineers to evaluate the feasibility of particular design option. I first heard of the Most-Likely-to-Happen factor about 7 years ago from Mark Blair of the Australian Space Research Institute whilst I was working on the AUSROC 2.5 project. Mark credits the concept to a gentleman by the name of Andrew Cheers (whom I have not met).
When working on aerospace projects it is inevitable for engineers to get carried away with exotic design solutions which, at least in the head of the engineer proposing them, are the most perfect and elegant solution to the problem. Every engineer who I ever worked with is guilty at least once in their career of proposing, and vehemently arguing for, a wacky design solution which for some reason or other completely defies reality. Perhaps it could be a special yet-to-be-designed widget (aka a "silver bullet") in the system block diagram that somehow delivers all the most challenging functions in a neat little box (probably made from the material unobtanium). It could also be a design option that makes use of a Commercial-Off-The-Shelf (COTS) component that does exactly what is needed but which costs an order of magnitude more than the project team can ever raise. In space projects another very important example is an idea that requires a component falling under technology export control laws.
Engineers use all sorts of 'design factors' to estimate the uncertainty of the physical processes that define the success of failure of a design. Unlike design factors, the Most-Likely-to-Happen factor is not a quantitative value. Rather it is a symbol or metaphor describing the process that should be followed when estimating the impact of those other all-important constraints on the project such as finances, politics, manpower, schedule, expertise, motivation, etc..
The tendency of engineers to propose creative new design solutions is an incredibly valuable thing. It is the engineers equivalent of a painter experimenting with new a brush technique or a sculptor trying a new base material for the first time. But just like in all creative professions, a successful project does not end with the creative idea. Some day the project must actually be realized, and this is where elegant creativity hits cold hard reality.
Since by necessity we engineers work in design teams, no matter how hard it hurts, we all have to be prepared to listen to that irritating fellow engineer who stubbornly keeps on pointing out that uncomfortable little pragmatic detail that makes our lovely little design solution impractical. Only by thoroughly discussing those details in a comprehensive and honest manner can the team ever succeed in identifying the design option that is indeed the "most likely to happen". In fact, I would suggest that those very discussions are where the design team truly adds value to the project.
The Most-Likely-to-Happen factor is an especially valuable concept to keep in mind in volunteer projects or projects that are poorly funded. This applies to any Google Lunar X PRIZE team that doesn't yet have millions of Dollars/Euros/RMB/Yen flowing in from investors to support their early engineering work, and let's face it, that's most of us!
