The 50% solution is based on the expected value (the solution with a 50% probability of achieving the same or better values for schedule, effort, cost, and reliability). Each of the inputs to an estimate - size, application complexity, and productivity - has some uncertainty associated with it. If any one of your input parameters (size, for instance) increases significantly from the original estimate, it makes sense that the actual outcome will vary from the estimated outcome.
To manage this uncertainty, SLIM-Estimate employs the concept of a risk-buffered solution. The estimate or “work plan” is a 50% probability solution. If the project is managed to this 50% solution, the risk charts should reflect alternative delivery dates at various levels of assurance. You can decide which date to promise to the customer based on your risk tolerance and your assessment of the amount of uncertainty in your inputs.
Essentially, the time difference between the 50% delivery date and the 90% (or whatever assurance level you choose) delivery date represents the amount of schedule buffer needed to account for the possibility that one or more of your input parameters will vary significantly from the estimate.
The alternate outcomes on each risk chart are based on the assumption that the 50% solution represents the work plan for the project. If the project is worked to another plan (the 70% assurance solution, for instance), the values on the risk charts will change.