time impact analysis - method

7/28/2019 Time Impact Analysis - Method

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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 resultant completion date. The

delaying event is then added and the programme rescheduled

again. The difference between the 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 of the event. This method predicts the

effect of an event on the completion date and is therefore a

prospective method of delay analysis. The predicted effect maynot 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 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 programme as the starting point for the analysis. The programme isthen updated with actual progress up to the start of each delay event and a note taken of the expected completion

date at that point in time. The delay event is then impacted and if the completion 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

J anuary 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.

Advantages

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 mosttime-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-ClarkCJ-0625

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