Earned Schedule Difference: Schedule Variance and Performance Index
PMBOK v8 Definition
Schedule Variance (SV) is a measure of schedule performance expressed as the difference between the earned value and the planned value. It represents the difference between the work completed to a point in time, usually the data date, and the work planned to be completed to the same point in time. The equation is SV = EV − PV.
Schedule Performance Index (SPI) is a measure of schedule efficiency expressed as the ratio of earned value to planned value. The equation is SPI = EV / PV.
Both metrics come from the Earned Value Analysis technique within the Monitoring and Controlling Process Group, specifically part of the Control Schedule and Control Costs processes.
Why It Matters for the Exam
Schedule variance and SPI appear frequently in PMI exam questions because they test your ability to interpret project health using earned value data. Questions typically present numerical values and ask you to determine whether the project is ahead, on, or behind schedule, or to calculate the percentage of schedule efficiency. These concepts are also commonly tested in scenario-based questions where you must recommend corrective actions based on the variance direction.
Key Points to Remember (for the exam)
- SV Formula: SV = EV − PV (Earned Value minus Planned Value)
- SPI Formula: SPI = EV / PV (Earned Value divided by Planned Value)
- SV Interpretation: Positive = Ahead of schedule; Neutral (zero) = On schedule; Negative = Behind schedule
- SPI Interpretation: Greater than 1.0 = Ahead of schedule; Exactly 1.0 = On schedule; Less than 1.0 = Behind schedule
- Common Confusion: SV measures the dollar value of schedule deviation, not time units. A negative SV means you have completed less work in dollar terms than planned, not necessarily that you are behind in calendar days.
- SPI as Percentage: An SPI of 0.85 means the project is performing at 85% of the planned schedule efficiency—only 85% of the work planned has been accomplished for the time elapsed.
- Data Date Reference: Both SV and SPI are calculated at the data date (the point in time when status is measured), not at project completion.
Typical PMI Exam Example
Scenario: A project has a Planned Value (PV) of $120,000 and an Earned Value (EV) of $95,000 at the data date. What is the Schedule Variance (SV) and Schedule Performance Index (SPI)?
Calculation: SV = $95,000 − $120,000 = −$25,000 (Negative = Behind schedule). SPI = $95,000 / $120,000 = 0.79 (Less than 1.0 = Behind schedule). The project is behind schedule, having completed only 79% of the work planned to date.
PMI Exam Traps
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Trap: Confusing SV with time-based schedule delay
- Reality: SV is expressed in monetary units (dollars, euros, etc.), not in days or weeks. A negative SV means less work was completed than planned in dollar terms, but it does not directly tell you how many days behind you are.
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Trap: Thinking SPI > 1.0 always means the project will finish early
- Reality: SPI > 1.0 at the data date means work is ahead of schedule so far, but future performance may change. SPI is a snapshot, not a final prediction.
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Trap: Using Actual Cost (AC) in SV or SPI calculations
- Reality: SV and SPI use only EV and PV. Actual Cost (AC) is used for Cost Variance (CV = EV − AC) and Cost Performance Index (CPI = EV / AC), not for schedule metrics.
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Trap: Forgetting that EV cannot exceed the authorized PV budget for a component
- Reality: The earned value measured cannot be greater than the authorized planned value budget for a work component. This prevents overstating progress beyond what was planned.
Important PMI Connections
| Related Concept | Relationship Type | Exam Attention Point |
|---|---|---|
| Planned Value (PV) | Input to SV and SPI | PV is the authorized budget assigned to scheduled work. SV and SPI both require PV as the baseline for comparison. |
| Earned Value (EV) | Input to SV and SPI | EV is the measure of work performed expressed in terms of the budget authorized. Both SV and SPI depend on EV as the actual progress measure. |
| Cost Variance (CV) | Complementary metric | CV = EV − AC measures cost performance. Exam questions often test SV and CV together to assess overall project health (schedule + cost). |
| Variance at Completion (VAC) | Forward-looking projection | VAC = BAC − EAC projects final cost variance. While SV looks backward to the data date, VAC looks forward to project completion. |
Quick Review Questions
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A project has EV = $50,000, PV = $45,000, and AC = $55,000. What are the SV and SPI values, and what do they indicate about schedule performance?
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If SPI is 1.15, what percentage of planned work has been accomplished relative to the work scheduled to be completed?
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A project manager reports SV = $0 at the data date. What does this indicate about the project's schedule status?
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Can a project have a negative SV but an SPI greater than 1.0? Why or why not?
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If EV = $80,000 and PV = $100,000, is the project ahead of or behind schedule? What is the SPI value?
PMBOK v8 Reference
Section 5.2.3.1 - Earned Value Analysis (Table 5-1: Earned Value Calculations Summary)
Note: This content is derived from PMBOK v8 Table 5-1, which covers Schedule Variance (SV), Schedule Performance Index (SPI), and related earned value metrics.