Current Period CPI in Earned Value 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="#why-period-cpi-matters-when-cumulative-cpi-already-exists">Why Period CPI Matters When Cumulative CPI Already Exists</a></li> <li><a href="#period-cpi-vs-cumulative-cpi-the-diagnostic-pair">Period CPI vs Cumulative CPI: The Diagnostic Pair</a></li> <li><a href="#worked-example-month-7-deep-dive">Worked Example: Month 7 Deep Dive</a></li> <li><a href="#when-to-use-period-cpi-vs-cumulative-cpi">When to Use Period CPI vs Cumulative CPI</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>Current period CPI is the <a href="/en/earned-value/definitions/cost-performance-index">Cost Performance Index</a> calculated for a single reporting period only. Not cumulative. Not smoothed. Just this month's earned value divided by this month's actual cost. It's volatile, it bounces around, and most teams ignore it because the numbers look alarming. That's exactly why you should be watching it.</p> <p><strong>Current Period CPI = Period EV / Period AC</strong></p> <p>Where period EV is the <a href="/en/earned-value/definitions/earned-value">earned value</a> generated in that reporting period alone, and period AC is the <a href="/en/earned-value/definitions/actual-cost">actual cost</a> recorded in that same period. Unlike <a href="/en/earned-value/definitions/cumulative-cpi">cumulative CPI</a>, which averages everything from day one, current period CPI reacts instantly to what happened last month.</p> <p>This term is part of the <a href="/en/earned-value/definitions">earned value definitions glossary</a>. For the full CPI and SPI breakdown, see the <a href="/en/earned-value/cpi-spi">CPI and SPI formulas page</a>.</p> <h2 id="why-period-cpi-matters-when-cumulative-cpi-already-exists">Why Period CPI Matters When Cumulative CPI Already Exists</h2> <p>Think of it like a heart rate monitor. Cumulative CPI is your resting heart rate averaged over a year. Period CPI is the reading right now, this second. Both matter. But if your heart rate spikes to 180 and you only look at the annual average, you're going to miss the cardiac event.</p> <p>I've seen this play out badly. A commercial team on a £45M water treatment project was reporting cumulative CPI of 0.96 every month. Comfortable. Green on the dashboard. Meanwhile, period CPI had dropped to 0.72, then 0.68 across two consecutive months. A major subcontractor was haemorrhaging money on the M&E package, but the cumulative average was masking it because the earlier months had been strong.</p> <p>By the time cumulative CPI finally dipped below 0.90, they were four months behind on the intervention.</p> <p>Period CPI is your early warning system. Cumulative CPI is your trend confirmation.</p> <h2 id="period-cpi-vs-cumulative-cpi-the-diagnostic-pair">Period CPI vs Cumulative CPI: The Diagnostic Pair</h2> <pre class="ge-ascii-diagram ge-anim"> PERIOD CPI vs CUMULATIVE CPI – 10-Month Tracking £18M Highway Widening, NEC4 Option C Month Period EV Period AC Period CPI Cum. CPI ────── ───────── ───────── ────────── ──────── 1 £410,000 £385,000 1.06 1.06 2 £520,000 £580,000 0.90 0.97 3 £610,000 £640,000 0.95 0.96 4 £680,000 £700,000 0.97 0.96 5 £550,000 £620,000 0.89 0.95 6 £720,000 £810,000 0.89 0.94 7 £480,000 £615,000 0.78 !!! 0.93 8 £690,000 £710,000 0.97 0.93 9 £730,000 £740,000 0.99 0.94 10 £600,000 £590,000 1.02 0.94 Period CPI Chart: 1.10 | * 1.00 | * * * 0.95 | * * 0.90 | * * 0.85 | 0.80 | * ← ALARM BELL 0.75 | └──┬───┬───┬───┬───┬───┬───┬───┬───┬── 1 2 3 4 5 6 7 8 9 10 Cumulative CPI Chart: 1.10 | * 1.00 | 0.95 | * * * * 0.90 | * * * * 0.85 | └──┬───┬───┬───┬───┬───┬───┬───┬───┬── 1 2 3 4 5 6 7 8 9 10 PERIOD CPI catches the month 7 problem IMMEDIATELY. CUMULATIVE CPI barely registers it – drops from 0.94 to 0.93. </pre> <p>Month 7 is the story here. Period CPI crashed to 0.78 because a subcontractor invoice landed that was £135,000 higher than the value of work delivered. That's a 22% overspend in a single month. The cumulative CPI? It moved from 0.94 to 0.93. Barely a ripple.</p> <p>If the project controls team only watched cumulative CPI, they'd have shrugged and moved on. Period CPI screamed "something is wrong with this subcontractor package."</p> <h2 id="worked-example-month-7-deep-dive">Worked Example: Month 7 Deep Dive</h2> <span class="ge-worked-label">Worked Example</span> <div class="ge-callout ge-anim"> <p><strong>Scenario:</strong> £18M highway widening under NEC4 Option C. Month 7 of a 14-month programme. The earthworks and drainage packages are running concurrently.</p> <p><strong>What happened:</strong></p> <ul> <li>Earthworks subcontractor submitted an interim payment of £480,000</li> <li>Only £310,000 of earned value was generated against that package (measured by units of cut/fill completed)</li> <li>Drainage package earned £170,000 against £135,000 actual cost (on track)</li> </ul> <p><strong>Period CPI calculation:</strong></p> <ul> <li>Period EV = £310,000 + £170,000 = £480,000</li> <li>Period AC = £480,000 + £135,000 = £615,000</li> <li>Period CPI = £480,000 / £615,000 = <strong>0.78</strong></li> </ul> <p><strong>What this tells you:</strong> For every £1.00 spent in month 7, the project only earned £0.78 of value. The earthworks package is the problem. Its individual period CPI is £310,000 / £480,000 = 0.65.</p> <p><strong>What the team did:</strong> Investigated the earthworks subcontractor's payment. Found that the invoice included £120,000 of plant standing time during an unexpected utility diversion. This was actually a <a href="/en/earned-value/definitions/compensation-event">compensation event</a> under clause 60.1(5), the Client had provided incorrect information about utility locations.</p> <p><strong>Outcome:</strong> The commercial team notified the compensation event within 8 weeks (clause 61.3). The £120,000 was recovered as an adjustment to Defined Cost. Retrospectively, the "real" period CPI for month 7 was £480,000 / £495,000 = 0.97. The system worked, but only because someone noticed the 0.78 and asked why.</p> </div> <h2 id="when-to-use-period-cpi-vs-cumulative-cpi">When to Use Period CPI vs Cumulative CPI</h2> <p>Don't pick one. Use both. But use them for different decisions.</p> <div class="ge-table-wrap ge-anim"><table class="ge-table"> <thead> <tr> <th>Decision</th> <th>Use Period CPI</th> <th>Use Cumulative CPI</th> </tr> </thead> <tbody> <tr> <td>"Should we investigate this subcontractor?"</td> <td>Yes. It shows this month's efficiency</td> <td>No, too smoothed to spot package-level issues</td> </tr> <tr> <td>"What will the project cost at completion?"</td> <td>No, too volatile for forecasting</td> <td>Yes, stable base for <a href="/en/earned-value/definitions/estimate-at-completion">EAC</a> formulas</td> </tr> <tr> <td>"Is performance improving or declining?"</td> <td>Yes, 3-month trend of period CPI shows direction</td> <td>Partially, but lags by several months</td> </tr> <tr> <td>"Do we need a recovery plan?"</td> <td>Yes, consecutive periods below 0.85 demand action</td> <td>Yes, if cumulative drops below 0.90 past 20% complete</td> </tr> <tr> <td>"What do we tell the board?"</td> <td>Context only, "last month was unusually low because."</td> <td>Lead metric, "project efficiency is tracking at 0.93"</td> </tr> </tbody> </table></div> <h2 id="common-mistakes">Common Mistakes</h2> <ol> <li><strong>Ignoring period CPI because it's volatile</strong>: Yes, it bounces. That's the point. A cumulative CPI that drops 0.01 started with a period CPI that dropped 0.15. If you wait for the cumulative to move, you've lost months.</li> <li><strong>Panicking over a single bad month</strong>: One period CPI of 0.75 doesn't mean the project is failing. It means something happened that month. Investigate, understand, decide. Two consecutive months below 0.80? Now you've got a pattern.</li> <li><strong>Using period CPI in EAC formulas</strong>: Don't. Period CPI is far too noisy for forecasting. The standard <a href="/en/earned-value/eac-etc-tcpi">EAC formula</a> uses cumulative CPI for a reason. Period CPI would produce an EAC that swings by millions every month.</li> <li><strong>Not breaking it down by control account</strong>: Project-level period CPI of 0.88 could mean every package is slightly underperforming, or one package is at 0.50 and the rest are fine. The <a href="/en/earned-value/definitions/control-account">control account</a>-level breakdown is where the diagnostic value lives.</li> <li><strong>Comparing period CPI across different projects</strong>: Period CPI is highly context-dependent. A month with a major concrete pour will have different cash flow dynamics than a month of finishing works. Compare period CPI across time on the same project, not across different projects.</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, giving you accurate earned value data without manual spreadsheet updates. <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 a good current period CPI?</h3> <p>There isn't a universal "good" number. Anything between 0.90 and 1.10 is generally healthy for a single reporting period on a construction project. The real signal is in the trend. Three consecutive months above 0.95? Solid. Three consecutive months below 0.85? You've got a systemic problem, not a blip.</p> <h3>How often should you calculate period CPI?</h3> <p>Monthly, aligned with your cost reporting cycle and subcontractor payment applications. Some larger programmes calculate it fortnightly, but monthly is the standard in UK construction. The key is consistency, always use the same period length so you're comparing like with like.</p> <h3>Can period CPI be greater than 1.0?</h3> <p>Absolutely. A period CPI of 1.08 means you earned 8% more value than you spent that month. This can happen when planned work completes ahead of budget (genuine efficiency), when subcontractor invoices lag behind physical progress (timing difference), or when previously overspent packages are corrected. Just as with low period CPI, interrogate the number before celebrating.</p> <h3>Should I report period CPI to the client?</h3> <p>On NEC4 Option C and D contracts, where open-book cost reporting is the norm, yes. It demonstrates a mature project controls approach and helps the Project Manager understand your efficiency trends. On lump sum contracts, your internal reporting should track it but you'd typically report cumulative CPI externally, if at all.</p> </article> </div>