Time & Material
Time & Material does not follow a fixed model in which the deliverables and price are fixed upfront. Instead, it allows you to budget based on the amount of time actually spent and the number of engineers who actually work on a project. This engagement model is best when the specifications have still not been decided, or for development using an agile approach in which the specifications are derived or change during the development.
Notes on using the Time & Material model
Unlike the Fixed Price model for which the price and schedule is decided upfront, the Time & Material engagement may result in unexpected costs if the development takes longer than planned. On the other hand, the cost could be lower than budgeted if the development finishes ahead of schedule.