CPI vs SPI: Reading Cost and Schedule Performance Together

Cost Performance Index (CPI) and Schedule Performance Index (SPI) are two earned value management ratios that answer different questions about the same project. CPI asks how efficiently the project is converting actual cost into earned value. SPI asks how much value has been earned relative to the value that was planned by the status date.

The CPI and SPI formulas are simple: CPI = EV / AC and SPI = EV / PV, where EV is earned value, AC is actual cost, and PV is planned value. Interpreting them well is harder because each ratio compresses a project condition into one number. A ratio below 1.0 signals unfavorable performance, but it does not explain the cause, the remaining work, or the corrective action.

For candidates working toward the PMP exam or PMP certification, the important skill is not just calculating the ratios. It is reading what combinations of CPI and SPI imply and knowing when additional schedule, cost, scope, and risk evidence is required.

Earned value connects scope, schedule, and cost through a common value scale

Traditional project reporting can say that a team spent 60% of the budget and used 60% of the planned time without answering how much planned work was actually completed. Earned value adds that missing measure by assigning budgeted value to completed work.

Planned Value represents the budgeted value of work that should be complete by the status date. Earned Value represents the budgeted value of the work actually completed. Actual Cost represents what the project actually spent to accomplish that work.

Those three measures create a common language. CPI compares EV with AC, while SPI compares EV with PV. The project can therefore be efficient on one dimension and inefficient on the other.

CPI measures cost efficiency, not simply whether total spending is high

Suppose a project has earned $80,000 of budgeted value and spent $100,000. CPI is 0.80. The project is receiving 80 cents of earned value for each dollar of actual cost, relative to the approved performance baseline.

A CPI above 1.0 is favorable cost efficiency; a CPI of 1.0 is on plan; a CPI below 1.0 is unfavorable. But a high CPI does not automatically mean the project will finish under budget. Future work may be more expensive, assumptions may change, or early savings may be offset later.

CPI is most useful when combined with trend, forecast, and explanation. A small one-period fluctuation means something different from a sustained decline across several reporting cycles.

SPI measures schedule efficiency in earned-value terms

Suppose planned value is $100,000 and earned value is $75,000. SPI is 0.75. By the status date, the project has earned 75% of the value it had planned to earn.

An SPI above 1.0 is favorable; 1.0 is on plan; below 1.0 is unfavorable. However, SPI does not tell you that the project is “25% late” in calendar time. The ratio compares earned value with planned value, not days with days.

This distinction prevents a common mistake. A project with SPI 0.80 may not be exactly 20% behind its finish date. You still need the network schedule, critical path, float, milestone status, and the distribution of remaining work to understand time impact.

CPI and SPI can point in opposite directions

Four combinations are possible. CPI above 1.0 and SPI above 1.0 is favorable on both dimensions. CPI below 1.0 and SPI below 1.0 is unfavorable on both. The more interesting cases are mixed.

If CPI is above 1.0 while SPI is below 1.0, the project is earning value efficiently in cost terms but more slowly than planned. Perhaps the team is understaffed, a dependency is late, or a deliberate cost-saving choice reduced throughput.

If CPI is below 1.0 while SPI is above 1.0, the team is progressing faster than planned but paying more to do so. Overtime, premium freight, added contractors, or expensive acceleration may explain the result. The correct management response differs from a project that is simply late and over budget.

Variance and index tell the same condition in different forms

Cost Variance is EV – AC. Schedule Variance is EV – PV. The signs align with the indexes: a negative cost variance generally accompanies CPI below 1.0, and a negative schedule variance accompanies SPI below 1.0.

Indexes are useful for proportional comparison. A CPI of 0.90 can be compared across projects more easily than a cost variance of negative $50,000 when project sizes differ. Variances are useful when stakeholders need the magnitude in currency or budgeted value.

Use the representation that helps the decision. Executives may need both: the index to show efficiency and the variance to show financial or schedule-value magnitude.

CPI can feed forecasts, but forecasts are assumptions expressed mathematically

Earned value provides several Estimate at Completion formulas depending on what the project team assumes about future performance. If current cost efficiency is expected to continue, one forecast may use BAC / CPI. If the original remaining estimate is still valid, another approach can use AC + (BAC – EV). If both cost and schedule efficiency are expected to influence remaining work, a different denominator may be appropriate.

The formula is not the decision. First decide which assumption is defensible. Is the cost overrun caused by a one-time event? Has the estimating basis changed? Will future work resemble completed work? Is the remaining work on the critical path more expensive than earlier work?

A sophisticated forecast states the assumption and sensitivity rather than presenting one number as inevitable.

SPI has an important limitation near project completion

Schedule performance measured through EV and PV behaves differently as the project approaches the planned finish. Once all planned value has accumulated, PV reaches the budget at completion. As work eventually finishes, EV also reaches that value, which drives SPI toward 1.0 even if the project completed late.

That is why SPI should not replace schedule analysis. As the project matures, critical-path status, milestone dates, forecast finish, float, and time-based schedule measures become increasingly important.

An exam question can test this concept by offering SPI as if it were a complete calendar forecast. Remember what the denominator represents and what it does not.

The ratios are indicators, not root causes

A CPI of 0.82 tells you there is unfavorable cost efficiency. It does not tell you why. The cause might be inaccurate estimates, scope change, rework, procurement cost, productivity problems, exchange rates, technical defects, or intentional schedule acceleration.

Likewise, a weak SPI could come from a critical supplier delay, underestimated work, resource contention, late approvals, rework, sequencing mistakes, or a noncritical activity that carries high planned value.

Management should investigate the driver before selecting a response. Treating the ratio itself as the problem can lead to superficial corrective action.

Baseline quality determines metric quality

Earned value depends on an integrated, credible performance baseline. If scope is vague, work is not time-phased sensibly, or budgets are disconnected from actual work packages, CPI and SPI can produce mathematically precise but operationally misleading output.

Change control matters for the same reason. Approved scope changes may require baseline updates. Quietly rewriting the baseline to make performance look better destroys the measure’s usefulness.

Good earned value practice therefore depends on scope definition, scheduling, estimating, work authorization, progress measurement, and disciplined change management—not only on spreadsheet formulas.

Use thresholds as investigation triggers, not automatic verdicts

Organizations may define thresholds such as CPI or SPI below 0.90 requiring attention. Thresholds can standardize escalation, but they should not replace judgment. A small short project and a billion-dollar program may require different sensitivity.

Look at trend and forecast. Is the ratio improving? Is a one-time event rolling out of the reporting period? Is the variance concentrated in one control account? Is the problem on the critical path? Does the current recovery plan change the expected result?

Good reporting pairs the number with a concise explanation, owner, corrective action, and forecast impact.

Practice interpretation before speed

Build small examples with EV, PV, and AC, then calculate CPI, SPI, cost variance, and schedule variance. Before touching the calculator, predict whether each result should be favorable or unfavorable. This catches formula swaps immediately.

Then create mixed scenarios. A project is under budget but behind schedule. Another is ahead of schedule because it paid overtime. Another has a weak CPI due to a one-time equipment purchase but expects normal future performance. Ask which forecast assumption fits each case.

The deeper project-management context matters too. project risk management helps explain why performance indicators should trigger investigation into uncertainty and response rather than be read in isolation.

Explain CPI and SPI in decision language

Stakeholders do not always need the full earned value vocabulary. “We are spending faster than the value we are earning, primarily because rework increased labor cost” is more useful than “CPI is 0.83” on its own. “We are behind the planned value curve, but the delayed work has float and is not yet changing the forecast finish” is more precise than saying “SPI is bad.”

Keep the metrics available for rigor, then translate them into cause, impact, and action. The ratio is evidence. The management value comes from what the team does with it.

CPI and SPI work best as a pair because projects can be efficient in cost and inefficient in schedule, or the reverse. Read both, test the underlying data, inspect the schedule and cost drivers, and forecast with assumptions you can defend.