Sizing is arguably the most challenging part of any software estimate. Without a notion of functional size, managers may find it difficult to negotiate realistic schedules based on their demonstrated ability to deliver software. They are unable to show empirically why the twelve person team that worked so well on a 150,000 ESLOC project over six months not only fails to deliver a 75,000 ESLOC project in half the time, but produces an error-ridden product that infuriates the customer. Unlike manufacturing shoes, software development is full of non-linear relationships between size, time, effort, and defects. What data driven estimation does successfully is arm managers with the ability to sanity check their current plans against past performance and negotiate achievable outcomes based on a realistic assessment of how much functionality can be built with a set time frame and resource profile.
So how do project managers get a better handle on size? The best place to start is with establishing a practical method for size estimation.