Limited Days Estimation (LDE) is an agile estimation technique that has been developed to support agile IT development and delivery. LDE is an alternative to Story Point or Relative Size estimation, and has been successfully applied in conjunction with XP, Scrum, Kanban and other agile methodologies. LDE works best when applied to estimation at the point of requirements breakdown into work items rather than estimation across an entire requirement inventory or product backlog.
For estimation across an entire requirement inventory or product backlog we would suggest using Dynamic Range Estimation (DRE) and then applying LDE for requirements breakdown prior to development and implementation. The two estimation techniques – Dynamic Range and Limited Days – when used together can reduce the overhead of estimation and provide fast feedback and visibility into size of work and associated risk.
There are three factors that put the ‘limited’ into Limited Days Estimation; the following ‘quick tour’ will describe them all, but at a glance they are:
1) Work in Progress
2) Minimum Breakdown
3) Maximum Size
We will begin by describing the work in progress limit. For the purpose of estimation we assume a work in progress limit of one, and we express that work in progress limit as a dedicated team day. The dedicated team day is the basic unit of estimation for all Limited Days estimates. The Limited Days method defines a dedicated team day as:
‘The amount of time for a team to complete a work-item described in days, if the team dedicates itself to completion of that single work-item and nothing else’.
It is important to note that the estimate is not described in man-days. With the Limited Days method we specifically do not break down tasks and effort into the granularity of either man-days or man-hours. Completion of work is considered to be a team responsibility, and estimates are therefore provided and recorded at team level.
The Limited Days method was conceived to avoid the overhead of estimation at a level that is so ‘finely grained’ that it ceases to represent a return on investment for time spent. With LDE we adopt the stance that, when estimating at the level of tasks and man-hours, the increased transaction cost associated with estimation does not return any increased benefit towards predictive delivery. An estimate that is expressed in individual man-hours may be more precise but increased precision does not describe any greater degree of accuracy. To this end, Limited Days defines the minimum breakdown limit. The minimum breakdown limit equates to half a dedicated team day and is expressed as follows:
‘Any work-item that is estimated to be equal to or less than 0.5 dedicated team days requires no further breakdown before development or implementation begins’.
Having established the minimum breakdown limit to prevent overestimation, we must now also establish the maximum size limit. Limited Days Estimation provides a specific definition for the maximum size limit:
‘Any work-item that is estimated to be greater than 5 dedicated team days must be broken down to smaller work-items before development or implementation begins’.
The maximum size limit is included with three specific goals in mind:
1) We want to ensure fast feedback regarding the accuracy and validity of our estimates. By creating the maximum size limit we ensure that no work is estimated to take longer than five dedicated team days before it is complete. If we then dedicate the team’s efforts to any given work item, even the largest should take no longer than one working week to complete. This ensures a responsive feedback loop regarding the accuracy of estimation and the opportunity to learn quickly and revise estimates if necessary.
2) We want to ensure fast feedback that validates our understanding of the requirements from which we have formed our estimates. Any estimates based upon a misunderstanding of requirements must be considered suspect and prone to inconsistency when measured against actual time required for development or implementation.
3) We want to ensure some degree of consistency regarding the size of work items for which the team are providing estimates. Whether you measure the size of requirements based upon complexity or effort to implement one characteristic seems consistent. As the size of the estimate increases, the level of inaccuracy within that estimate also increases. The maximum size limit helps constrain these levels of inaccuracy to a level that facilitates consistent delivery.
This has been a brief introduction to LDE and some of the underlying principles that make it such a successful agile estimation technique. For a more detailed view and guidance on how to conduct an estimation workshop, check out the ‘Getting Started with Limited Days’ section of this site.