Milestone reports are a succinct way of advising top management of the status of the project and should act as a spur to the project team to meet these important deadlines. This is especially important if they relate to large tranches of progress payments.

Milestones are marked on bar charts or networks by a triangle or diamond and can be turned into a monitoring system in their own right when used in milestone slip charts, sometimes also known as trend charts.

Fig. 25.1 shows such a slip chart, which was produced at reporting period five of a project. The top scale represents the project calendar, and the vertical scale represents the main reporting periods in terms of time. If both calendars are drawn to the same scale, a line drawn from the top left-hand corner to the bottom right-hand corner will be at 45 degrees to the two axes.

The pre-planned milestones at the start of the project are marked on the top line with a black triangle (▼).

As the project progresses, the predicted or anticipated dates of achievement of the milestones are inserted so that the slippage (if any) can be seen graphically. This should then prompt management action to ensure that the subsequent milestones do not slip! At each reporting stage, the anticipated slippages of milestones as given by the programme are re-marked with an × while those that have not been re-programmed are marked with an O. Milestones that have been met will be on the diagonal and will be marked with a triangle (∇).

As the programmed slippage of each milestone is marked on the diagram, a pattern emerges, which acts not only as a historical record of the slippages, but can also be used to give a crude prediction of future milestone movements.

A slip chart showing the status at reporting period 11 is shown in Fig. 25.2. It can be seen that milestone A was reached in week 22 instead of the original prediction of week 16. Milestones B, C and D have all slipped, with the latest prediction for B being week 50, for C being week 62, and for D being week 76. It will be noticed that before reporting period 11, the programmed predictions are marked X, and the future predictions, after week 11, are marked O.

If a milestone is not on the critical path, it may well slip on the slip chart without affecting the next milestone. However, if two adjacent milestones on the slip chart are on the critical path, any delay on the first one must cause a corresponding slippage on the second. If this is then marked on the slip chart, it will in effect become a prediction, which will then alert the project manager to take action.

Once the milestone symbol meets the diagonal line, the required deadline has been achieved.

The quickest way to explain how this planning method works is to follow a simple example involving the construction of four identical, small, single-storey houses of the type shown in Chapter 47, Fig. 47.1. For the sake of clarity, only the first five activities will be considered, and it will be seen from Fig. 47.2 that the last of the five activities, E – ‘floor joists’, will be complete in week 9.

Assuming one has sufficient resources and space between the actual building plots, it is possible to start work on every house at the same time and therefore finish laying all the floor joists by week 9. However, in real life this is not possible, so the gang laying the foundations to house No. 1 will move to house No. 2 when foundation No. 1 is finished. When foundation No. 2 is finished, the gang will start No. 3, and so on. The same procedure will be carried out by all the following trades, until all the houses are finished.

Another practical device is to allow a time buffer between the trades so as to give a measure of flexibility and introduce a margin of error. Frequently such a buffer will occur naturally for such reasons as hardening time of concrete, setting time of adhesive, or drying time of plaster or paint.

Table 47.1 can now be partially redrawn showing the buffer time, which was originally included in the activity duration. The new table is now shown as Table 25.1.

Fig. 25.3 shows the relationship between the trades involved. Each trade (or activity) is represented by two lines. The distance between these lines is the duration of the activity. The distance between the activities is the buffer period. As can be seen, all the work of activities A to E is carried out at the same rate, which means that for every house, enough resources are available for every trade to start as soon as its preceding trade is finished. This is shown to be the case in Fig. 25.3.

However, if only one gang is available on the site for each trade, e.g., if only one gang of concreters laying the foundations (activity B) is available, concreting on house two cannot start until ground clearance (activity A) has been completed. The chart would then be as shown in Fig. 25.4. If instead there were two gangs of concreters available on the site, the foundations for house two could be started as soon as the ground has been cleared.

Building the dwarf wall (activity C) requires only 1.9 weeks per house, which is a faster rate of work than laying foundations. To keep the bricklaying gang going smoothly from one house to the next, work can only start on house one in week 7.2, i.e., after the buffer of about 2.5 weeks following the completion of the foundations of house 1. In this way, by the time the dwarf walls are started on house 4, the foundations (activity B) of house four will just have been finished. (In practice of course there would be a further buffer to allow the concrete to harden sufficiently for the bricklaying to start.)

As the oversite concreting (activity D) only takes 0.9 weeks, the one gang of labourers doing this work will have every oversite completed well before the next house is ready for them. Their start date could be delayed if necessary by as much as 3.5 weeks, since apart from the buffer, this activity (D) has also 1 week float.

It can be seen therefore from Fig. 25.4 that by plotting these operations with the time as the horizontal axis and the number of houses as the vertical axis, the following becomes apparent.

If the slope of an operation is less (i.e., flatter) than the slope of the preceding operation, the chosen buffer is shown at the start of the operation. If, on the other hand, the slope of the succeeding operation is steeper, the buffer must be inserted at the end of the previous operation, since otherwise there is a possibility of the trades clashing when they get to the last house.

What becomes very clear from these diagrams is the ability to delay the start of an operation (and use the resources somewhere else) and still meet the overall project programme.

When the work is carried out by trade gangs, the movement of the gangs can be shown on the LoB chart by vertical arrows as indicated in Fig. 25.4.