What is a burn down chart?
A burn down chart is an important tool in scrum. It provides a visual representation about the progress achieved in a sprint while it is underway. They are very common and extensively used by scrum masters while scrum is being implemented in a project. The quantity, or the amount of work remaining, in the form of pending tasks, is typically exhibited in a burn down chart. The chart is simple and easy to understand, even by people who are not familiar with scrum methodology. Burn down charts are very useful for estimation purposes, and are essential for determining the sprint velocity – the rate at which work in the form of user stories is being completed by the development team – and planning the sprint release.
Plotting the burn down chart
A burn down chart can be plotted by including the work remaining in the form of story points along the vertical Y-axis and the working days along the horizontal X-axis. The pending work is typically represented in story points – a unit of measurement to calculate the importance and priority of user stories in the sprint backlog – instead of user stories. The reason is user stories are broken down into tasks during the second half of the sprint planning meeting by the development team. It becomes difficult to read and understand the chart if tasks are represented along the Y-axis. User stories are descriptive in nature, and do not have a number or a value associated with them, so it becomes difficult to estimate them. Therefore, the story points, which are numeric values associated with each user story, are used for plotting purposes Scrum certification.
On the first day of the sprint, the total number of days completed in the sprint is zero while the pending work constitutes the entire sprint backlog. Conversely, when the entire sprint is processed, the number of days is equivalent to the days the sprint has actually lasted – generally two weeks – and the pending work should be zero. The burn down chart can include an “ideal” working line, which originates from the top left-hand-side corner of the graph where the number of days is equivalent to zero along the X-axis and the story points are maximum on the Y-axis. The ideal line extends or runs diagonally, and connects to the bottom right-hand-side of the graph, on the X-axis where the days are maximum i.e. equal to the total number of days the sprint lasts. The story points are zero on the Y-axis. The ideal line reflects the desired level of work and sprint completion status.
Starting from day zero, when the story points are maximum, as the sprint progresses, the days increase along the X-axis while the story points start decreasing along the Y-axis. On day one, when some of the user stories are completed, a point is interpolated on the graph which correlated to the first day of the sprint on the X-axis and a value equal to the maximum story points in the sprint minus the value of story points completed on that day. For example, if 25 story points are included in the sprint, and on day one of the sprint if work equal to 5 story points is completed, the total pending work remaining in the sprint is equivalent to 25 – 5 = 20 points. The coordinates of the point would be (0, 20) on the graph. Similarly, on the second day of the sprint, if work equal to 1 story point is completed, the total work remaining is equivalent to 20 – 1 = 19 story points. The coordinates for the second day would be (1, 19) on the graph. Many scrum masters prefer the dates to be displayed along the X-axis in lieu of days to make the chart more readable.
Reading the chart
Many burn down charts actually have the “ideal” line plotted on them – for reference and estimation purposes. It is a good visual indication whether the particular sprint is proceeding as per schedule, or whether the development work is carried out behind schedule. The line represents the ideal “burn down” required for attaining the sprint goal or the ideal position to be in at the end of sprinting day. The actual work carried out is plotted on the graph with the help of coordinate points. All the points plotted on the graph are connected with a line, which passes through them. If the actual line extends above the ideal line, it indicates that the sprint is proceeding behind schedule, and the required number of tasks is not being completed as per the sprint planning. This basically indicates two things – one, the team has to put in more efforts to compensate for the pending work which should be completed the next working day, and two – the team is required to introspect and find out why the tasks were not completed on time. It is important to self-correct and ensure the same mistakes are not repeated since scrum advocates self-learning and self-correction processes.