Commentary On The Benefits Of XP: Schedule Slips
Schedule slips: XP calls for short release cycles, a few months at most, so the scope of any slip is limited. Within a release, XP uses one- to four-week iterations of customer-requested features for fine-grained feedback about progress. Within an iteration, XP plans with one- to three-day tasks, so the team can solve problems even during an iteration. Finally, XP calls for implementing the highest priority features first, so any features that slip past the release will be of lower value. (Extreme Programming Explained, Chapter 1)
Response:
Projects have beginning and an ending (hopefully!), whether I as a consultant write the proposal, or I and my team, as employees, write the proposal. The schedule in the proposal is based on the input received from the customer (as defined by TQM). If the proposal is accepted, among other things, the customer will plan activities based on the anticipated completion date. In certain industries (such as the gaming industry) this is a very critical date and slipping on the completion date can kill a product.
Releasing a product in short release cycles by its very nature reduces the perceived schedule slip, however this schedule slip continues to aggregate for each release and the end result is the same--the product slips its schedule, whether it was done in one big release or many little ones. The only way to modify a project that is incurring schedule slip is:
- reduce the feature set
- work more efficiently
- work harder (overtime)
- make it up somewhere else
- add more people
- negotiate an extension
From the perspective of the customer, none of these are viable solutions except "work more efficiently". All the other options incur costs and penalties that, while difficult to quantify, reduce the profit potential of the product. The "make it up somewhere else" option does not necessarily involve additional costs but carries risk--the areas where the "make up" is anticipated to occur may not be realized.
XP's technique of pushing features that slip past the release date to a lower priority eventually results in a bottleneck of low priority items. As development proceeds, some of these items will enter the critical path to complete higher priority items which will result in a cascade effect of further schedule slips. This will manifest itself in the Iteration Planning Meeting, as low priority items enter the critical path of high priority stories and tasks.
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