Question

How does changing a case estimate affect EBS? I think I know and am going to answer this one myself so I have a place to find it later. :-)

Answer 1

EBS (Evidence Based Scheduling) is concerned with two important case states.

1. Not worked on. (Time has not been logged)
2. Worked on. (Time has been logged)

Not Worked On
Changing the estimate when time has not been logged simply changes the estimate and is reflected in the EBS projected 50% date. To the extent updated estimates more closely match actual time worked the standard deviation of future EBS projections will decrease.

Worked On
When a case is worked on, even if only 1 minute is tracked, FogBugz locks the Current Estimate in a field called Original Estimate. Changing the current estimate after work has begun immediately impacts the EBS projected 50% date. However, the Original Estimate is locked in. To the extent updated estimates more closely match actual time worked the standard deviation of future EBS projections will increase.

Both scenarios are good as they give accurate insight into project completion in real time and increase the accuracy of EBS projections by using realistic velocities (Original Estimate / Actual Elapsed Time) in future modeling scenarios.

For me, knowing FogBugz locks estimates after logging time is an important insight. When implementing an agile development process where estimation is done up front and estimation accuracy needs to be reflected in future projections; FogBugz gets it right. Where EBS adds tremendous value is in providing real-time insight into the expected end date of the current sprint/milestone/project.

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From FogBugz help Entering Estimates in FogBugz

You can change estimates as many times as you want, as you learn more about the work required for a task. However, as soon as you start working on a case (using time-sheets or the working on menu) FogBugz takes a snapshot of the current estimate and stores it permanently as the original estimate.

From FogBugz help How To Estimate Software Tasks

For each completed feature, EBS looks at the original estimate entered and the actual time elapsed. It divides estimate by elapsed to get the velocity of that feature's implementation.

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Answer 2

See Effect on EBS of Changing Assigned-to and Current-estimate, plus Help -> FogBugz Help -> (Project Management and Schedules) Evidence-Based Scheduling

Amongst the variables that EBS uses to calculate its probabilities there is something like a historical accuracy of the estimator but there must also be current estimated time remaining. The current estimated time remaining must come from the most recent estimate, but from discussions here I know that the historical accuracy of the estimator calculation ignores any changes in estimates and only uses the estimate that is active when a developer first indicates "Working on" that case.