If you have a task constraint generating negative float then you will want a fast and insightful way to view its impact on the schedule. Displaying negative float on the Gantt chart is the best way to achieve both speed and clarity. Here we demonstrate how to display negative float on the Gantt chart in Microsoft Project 2013.
The task table is good for listing the total float, i.e. total slack, of tasks. However, the Gantt chart is at the heart of scheduling software, and displaying total slack on the Gantt chart provides a much quicker and, possibly, more comprehensive graphical presentation. In particular, we want to display the negative slack of tasks on the Gantt chart. In this way we can better understand the impact of our schedule’s task constraints and resulting negative slack directly from the Gantt chart.
This article demonstrates how to display negative slack on the Gantt chart in Microsoft Project 2013.
We have in Figure 1 our demonstration project.
Note, in particular the total slack column that shows we have some tasks with negative slack, zero slack, or positive slack. Displaying a positive float bar is quite simple you can toggle slack in the format tab and bar styles ribbon group, Figure 2.
If you want a more involved but more flexible approach to displaying positive float on the Gantt chart refer to the following blog Microsoft Project: Displaying the Total Float (Slack) of Tasks on the Gantt Chart.
We now demonstrate how to display negative slack on the Gantt chart. Before we proceed let’s take a look at what task constraint is generating this negative slack. In Figure 3 we see that we have a ‘Start No Later Than’ (SNLT) constraint on erect steel structures that is causing the negative float.
Great! Now that we know the cause of the negative slack we want to examine its impact via the Gantt chart. To do this we need to create a new bar for negative float. Select bar styles from the format drop down menu in the format tab and bar styles ribbon group, Figure 4.
In the bar styles dialog click insert row to add a new bar definition. Name it the ‘negative float bar’, Figure 5.
Adjust the start and middle graphical definitions of the negative float bar so that its appearance is similar to our negative float bar displayed in Figure 5.
In the ‘show for … tasks’ or filter column choose the normal filter, Figure 6.
The Gantt chart thus will only display negative float on normal tasks. Milestone tasks are therefore excluded.
The next step in creating a negative float bar is important to input correctly. We want the ‘From’ of our negative float bar definition to be ‘negative slack’, and not ‘task start’. Make ‘To’ equivalent to ‘task start’, Figure 6.
If you do not set the ‘From’ and ‘To’ fields correctly Microsoft Project will not display negative slack bars as intended. So the definition of our negative float bar is from left to right on the timescale or from the negative slack date to the task start date.
Figure 7 displays the Gantt chart with negative float bars.
Our SNLT constraint on ‘erect steel structures’ is generating negative float on ‘erect steel structures’ and several other successor tasks. In Figure 8 we adjust the timescale for a closer look at the negative float.
We see in Figure 8 that the 3-day negative float bars displayed on the Gantt chart match the total slack column in the task table.
From the Gantt chart we know that we will currently miss the ‘erect steel structures’ SNLT task constraint date by three days. Negative float is displayed on every task that potentially can make up this lost time. Note that ‘set foundations’ and ‘dig cable trench’ start before ‘erect steel structures’, but have no effect on its float. Also, if you shift mobilize left to make time then your project start date must come earlier.
At the heart of project management software is the Gantt chart, which is a graphical presentation of the schedule. Leverage the power of the Gantt chart to analyze the impact of task constraints on the schedule.
The Gantt chart provides a quick way to gain insight and clarity on negative slack and the schedule. The trick to displaying negative float on the Gantt chart is to remember to define the negative slack bar from left to right on the timescale.