Time Impact analysis Date
28 June 2006
Judgment The Issue
The time impact analysis method of delay analysis involves updating the contractor’s master programme with percentage progress at the start of a delaying event. The programme is then rescheduled to determine the the resultant completion date. The delaying event is then added and the programme rescheduled again. The difference between the two two completion dates, if any, is the delay that is predicted to arise from the delaying event.
Implication
This approach takes account of actual progress and indicates the critical path at the time time of the event. This method predicts the effect of an event on the completion date and is therefore a prospective method method of delay analysis. The predicted effect may not precisely reflect what will actually happen, so this method is often described as entitlement based.
print
In this the fifth in my series of articles on delay analysis, I consider the time time impact method of delay analysis. This is an approach to modelling delay that relies upon the use of the contractor’s planned programme and the availability of progress data to determine the progress of the works at the time of a delaying event. This method provides a basis for determining the expected effect of an event on the completion date and is therefore a forward looking or prospective method of analysis. The method The method uses the contractor’s planned planned programme as the starting point point for the analysis. The programme is then updated with actual progress up to the start of each delay event and a note taken of the expected co mpletion date at that point in time. The delay event is then impacted and if the completion date date is extended then this is recorded as the critical effect of the impacted delay event. Each delay event is analysed chronologically and cumulatively. The outcome is a reflection of the activities and logic of that part of the planned programme that at that time remains to be completed. There is scope for updating the planned programme to reflect changes in planning that often occur during the life of a project – the ‘line in the sand’. It may also be necessary to adapt the logic of the programme to introduce the delay and properly model its expected effects as well as to allow for any potential mitigation that may be achieved through re-programming.
The graphic above shows a simple example. The yellow bars are the baseline and the delay event being modelled is a 10 day prolongation of activity 4 which originally had 5 days float. Progress is added as at 23 January showing that activity 4 was running later than planned using up 3 of the 5 days of float. The completion date remains as planned. The 10 day delay event 1 is then added that uses up the remaining 2 days of float and causes a critical delay to completion of 8 days. Selection criteria Where there is a good planned programme, reliable and consistent progress data as well as good as-built data and regular updates of the programme, then this method of analysis may be used. Progress data may not be good enough to properly assess progress prior to the impact of each event and so derivatives of this technique may be adopted such as ‘windows analysis’, which uses progress data at the intervals at which it is available such as two-weekly or monthly intervals, or ‘watershed analysis’ which adopts the method at key milestones such as completion of foundations, building watertight, power-on etc. Issues This is a prospective method of analysis that provides for the assessment of the expected effect of an event at the time at which it occurs. It is therefore sometimes described as an entitlement based approach. It does not attempt to model the actual effect of an event based on a retrospective view of the actual consequences but it looks at the likely effect of an event at the time it occurs. There may also be a further degree of speculation in the assessment of the potential consequential effects of an event. Where the analyst makes such assessments this may subjectively be based on an assumed course of events that may never occur. Despite its advantages (described below), it should be recognised that this method also has a subjective aspect to it which is that the planned programme yet to be progressed dictates the critical path at that time of the event and any such programme is always at best an approximation of the reality of how things are built, so there is the possibility that the actual critical path may not be the one modelled at the time of the event. Care must be taken to ensure that the planned programme to complete is reasonable and any obvious errors in the programme are corrected. Advantag es The method has the merit of considering the impact of an event based on a consideration of the actual as-built progress at the time of the event. This has the advantage, if the modelling of the delay impact is done properly, of showing the expected entitlement that arises from an individual event, without the picture being confused by a chain of subsequent intervening or supervening events. This method has the particular appeal of providing an indication of the critical path at the time of the event rather than the ultimate as-built critical path, and is therefore an appropriate method for determining the expected effect of an event on the completion date. Disadvantages Despite its merits, the method has many problems, not least is that as the Protocol (2002) warns, it is ‘the most time-consuming and costly when performed forensically’. The fact that this method predicts the expected effect of an event means that the analysis will be partly hypothetical. The result is heavily influenced by the quality of the base programme, which has to be checked and verified as being achievable, and may ultimately bear no resemblance to what actually happened. Since many construction programmes are not resourced, and there may be a variety of ways of programming the project, this verification exercise may itself be somewhat subjective.
A further problem is that the updates of the programme require adequate and consistent progress information at each update. If there is no such progress information or it is not reliably consistent or it is necessary to infer from the progress data that is available the state of progress specifically at the time of the event, then the effect of a delay may be more or less in reality than would result from the analysis. Further key factors are the way in which the events are impacted on the analysis and the extent to which any subsequent reprogramming or re-sequencing is reflected in that part of the planned programme that remains to be progressed. This ongoing tinkering with the logic at each update, which may be necessary to reflect the inadequacies in the base programme can make the steps taken by the analyst using this technique difficult to follow even for the trained eye. Further there are very significant problems with communicating the results. Two programmes are generally produced at each event, one that reflects progress before the delay is impacted and one after. So this can result in a myriad of iterations of the analysis. Thus despite its appeal this method is difficult to put across in a manner that can easily be followed.
- Rob Palles-Clark CJ-0625
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