the likely impact of a delay contemporaneously (at the time the events in question occurred) on the progress and completion of the works based upon that which was known ‘on the ground’ at that time. The basis for this methodology, whether applied before or after the delay effect has occurred, is to impact a specific and anticipated (forecast) delay onto a programme, usually the contractor’s as-planned programme or a progressed version of this, to model the theoretical effect of the delay to the project completion date. In light of this, this approach is commonly used and favoured by contractors to contemporaneously demonstrate what extension of time is due as a consequence of employer delay events, as the works progress, given it only requires an analysis of what critical delay could reasonably have been expected to occur. (ii) Retrospective analyses (the microsope) Alternatively a retrospective approach to delay analysis (such as ‘planned versus as-built’, ‘time slice’ or ‘windows’ methodologies) considers subsequent events, and other issues affecting progress, to determine the actual impact of delays on the works and whether the events in question actually caused delay to completion, as a matter of fact, when the progress of the work as a whole is considered. This approach seeks to identify what the actual delays to the project were by analysing how the works actually progressed. In doing so it investigates what actually happened, what the delays were and thereafter what caused the delay. In light of this it is often favoured by employers (or their representatives) given that it requires that the critical delay caused by the excusable delay needs to be established as a matter of fact. Therefore the choice between which approach to adopt is central to measuring the extension of time as well as the evidential burden required to establish entitlement. Do both approaches lead to the same result? Previously, in Walter Lilly & Company v Mackay & another [2012]3 , the Courts, in considering the use of prospective and retrospective delay analysis methodologies for determining the extension of time due, noted in dicta that “… if each approach was done correctly, they should produce the same result.” However, in passing judgement in this recent case, the Courts have now accepted that “…a prospective analysis - in other words considering the critical path at any particular point in time as viewed by those on the ground at that time - does not necessarily produce the same answer