What Is Earned Schedule? ES for Construction Explained

<div class="ge-article-wrapper"> <nav class="ge-toc" aria-label="Table of contents"> <p class="ge-toc-label">In this article</p> <ul class="ge-toc-list"> <li><a href="#how-earned-schedule-is-derived">How Earned Schedule Is Derived</a></li> <li><a href="#the-formulas">The Formulas</a></li> <li><a href="#why-spi-breaks-down-and-es-doesnt">Why SPI Breaks Down (And ES Doesn't)</a></li> <li><a href="#worked-example-30m-water-treatment-programme">Worked Example: £30M Water Treatment Programme</a></li> <li><a href="#when-to-use-earned-schedule">When to Use Earned Schedule</a></li> <li><a href="#common-mistakes">Common Mistakes</a></li> <li><a href="#frequently-asked-questions">Frequently Asked Questions</a></li> </ul> </nav> <article class="ge-article-body"> <p>Earned schedule (ES) is a time-based extension to <a href="/en/earned-value/definitions/earned-value-management">earned value management</a> that measures programme performance in units of time rather than cost. Developed by Walt Lipke in 2003, it fixes a fundamental flaw in traditional EVM: the <a href="/en/earned-value/definitions/schedule-performance-index">Schedule Performance Index</a> (SPI) becomes unreliable in the second half of a project because it always converges to 1.0 at completion, even on projects that finished catastrophically late.</p> <p>The core idea: if you're at month 12 and you've completed £18M of work, but the baseline says you should have reached £18M by month 10.3, then your earned schedule is 10.3 months. You're 1.7 months behind.</p> <p>For the full EVM framework, see the <a href="/en/earned-value">earned value management guide</a>.</p> <h2 id="how-earned-schedule-is-derived">How Earned Schedule Is Derived</h2> <p>ES uses the same <a href="/en/earned-value/definitions/planned-value">Planned Value</a> and <a href="/en/earned-value/definitions/earned-value">Earned Value</a> data you're already capturing. You find the point on the PV curve where cumulative PV equals your current cumulative EV. The interpolation is linear between the two baseline periods that bracket your current EV.</p> <h2 id="the-formulas">The Formulas</h2> <div class="ge-table-wrap ge-anim"><table class="ge-table"> <thead><tr><th>Metric</th><th>Formula</th><th>What It Tells You</th></tr></thead> <tbody> <tr><td>ES</td><td>C + (EV - PV_C) / (PV_{C+1} - PV_C)</td><td>How many months of baseline progress you've achieved</td></tr> <tr><td>SV(t)</td><td>ES - AT</td><td>Schedule variance in time units (months)</td></tr> <tr><td>SPI(t)</td><td>ES / AT</td><td>Schedule efficiency (below 1.0 = behind)</td></tr> <tr><td>IEAC(t)</td><td>PD / SPI(t)</td><td>Independent estimate of total project duration</td></tr> </tbody></table></div> <h2 id="why-spi-breaks-down-and-es-doesnt">Why SPI Breaks Down (And ES Doesn't)</h2> <p>Traditional SPI (EV/PV) converges to 1.0 at project end because both EV and PV converge to BAC. A £40M rail project running at SPI 0.78 at the halfway point showed SPI 0.91 by month 14 of 18, not because it was recovering, but because the maths was converging. It finished 5 months late. SPI at completion? 1.0. Completely useless.</p> <p>SPI(t) doesn't have this problem. If you finish 5 months late on an 18-month programme, SPI(t) = 18/23 = 0.78 at completion. It stays honest.</p> <h2 id="worked-example-30m-water-treatment-programme">Worked Example: £30M Water Treatment Programme</h2> <span class="ge-worked-label">Worked Example</span> <div class="ge-callout ge-anim"> <p><strong>Scenario:</strong> £30M water treatment programme under NEC4 Option C. 20-month baseline. At month 12, EV = £16.8M. PV at month 10 = £15.5M, PV at month 11 = £17.9M.</p> <p>ES = 10 + (16.8 - 15.5) / (17.9 - 15.5) = 10.54 months. SV(t) = 10.54 - 12 = -1.46 months behind. SPI(t) = 0.878. IEAC(t) = 20 / 0.878 = 22.8 months (projected 2.8 months late).</p> <p>This triggers an <a href="/en/earned-value/definitions/early-warning">early warning</a> under clause 16.1.</p> </div> <h2 id="when-to-use-earned-schedule">When to Use Earned Schedule</h2> <p>ES is most valuable in the second half of a project and on programmes longer than 12 months. Track both SPI and SPI(t) from the start.</p> <h2 id="common-mistakes">Common Mistakes</h2> <ol> <li><strong>Only switching to ES when SPI stops working.</strong> Track both from day one.</li> <li><strong>Applying ES to LOE work packages.</strong> Filter them out.</li> <li><strong>Ignoring the interpolation assumption.</strong> ES assumes linear PV growth between periods.</li> <li><strong>Confusing SV(t) with float.</strong> SV(t) tells you how far behind the baseline you are, not whether you've consumed float.</li> </ol> <div class="ge-product-note ge-anim"> <p><strong>How Gather helps.</strong> Gather's AI reads your site diaries daily and maps progress against your cost-loaded programme. <a href="https://gatherinsights.com/contact">Book a demo</a> to see it working on a live NEC4 project.</p> </div> <h2 id="frequently-asked-questions">Frequently Asked Questions</h2> <h3>What's the difference between SPI and SPI(t)?</h3> <p>SPI (EV/PV) measures schedule performance in cost terms. SPI(t) (ES/AT) measures it in time terms. In the second half of a project, SPI converges toward 1.0 regardless of actual delay, while SPI(t) remains accurate.</p> <h3>Can I use earned schedule on NEC4 contracts?</h3> <p>Absolutely. ES uses the same PV and EV data as standard EVM. There's nothing contract-specific about ES.</p> <h3>Is earned schedule widely used in UK construction?</h3> <p>Not as widely as it should be. It's more established in defence and aerospace. The IPA has been pushing for better schedule performance metrics, and ES is gaining ground on major frameworks.</p> </article> </div>