CPI vs SPI skaidrojums: Izmaksu un grafika veiktspēja EVM
Izmaksu veiktspējas indekss (CPI) un Grafika veiktspējas indekss (SPI) ir divas visbiežāk novērotās metrikas iekš Earned Value Management. Abi ir efektivitātes rādītāji, kuriem kā bāzes līnija ir 1.0 — bet tie mēra pilnīgi atšķirīgas lietas, un projekta vadītājs, kurš tos nepareizi nolasa, var izdarīt ļoti nepareizus secinājumus.
Šis raksts detalizēti izskaidro abus indeksus: kā tie tiek aprēķināti, ko nozīmē to vērtības, kā tie mijiedarbojas un ko darīt, kad tie atšķiras.
Formulas īsumā
| Indekss | Formula | Mēra | Laba vērtība |
|---|---|---|---|
| CPI | EV ÷ AC | Izmaksu efektivitāti | > 1.0 |
| SPI | EV ÷ PV | Grafika efektivitāti | > 1.0 |
CPI: Izmaksu veiktspējas indekss
CPI mēra, cik daudz budžetētās darba vērtības tiek piegādāts par katru iztērēto dolāru. CPI 0.85 nozīmē, ka par katru faktiski iztērēto
CPI vs SPI Explained: Cost vs Schedule Performance in EVM
The Cost Performance Index (CPI) and Schedule Performance Index (SPI) are the two most-watched metrics in Earned Value Management. Both are efficiency ratios that use 1.0 as the baseline — but they measure completely different things, and a project manager who misreads them can draw badly wrong conclusions.
This article explains both indexes in depth: how they are calculated, what their values mean, how they interact, and what to do when they diverge.
The Formulas at a Glance
| Index | Formula | Measures | Good value |
|---|---|---|---|
| CPI | EV ÷ AC | Cost efficiency | > 1.0 |
| SPI | EV ÷ PV | Schedule efficiency | > 1.0 |
CPI: Cost Performance Index
CPI measures how much budgeted work value is being delivered per dollar spent. A CPI of 0.85 means that for every $1.00 actually spent, only $0.85 worth of planned work has been completed — an 18% cost overrun rate.
CPI Interpretation Guide
| CPI Value | Status | What it means |
|---|---|---|
| > 1.0 | Under budget | Delivering more value than each dollar costs |
| = 1.0 | On budget | Delivering exactly as planned per dollar |
| 0.9 – 0.99 | Slight overrun | Monitor closely; corrective action may be needed |
| < 0.9 | Over budget | Significant cost problem; management action required |
Key research finding: Studies show that once CPI drops below 0.9 after the 20% project completion point, it very rarely recovers. Early CPI values are strong predictors of final project cost performance.
SPI: Schedule Performance Index
SPI measures how efficiently the project is progressing through its planned work. An SPI of 0.80 means the team is completing only 80% of the work that was scheduled to be done by this point — the project is running 20% behind its timeline.
SPI Interpretation Guide
| SPI Value | Status | What it means |
|---|---|---|
| > 1.0 | Ahead of schedule | More work done than was planned for this period |
| = 1.0 | On schedule | Exactly on the planned timeline |
| 0.9 – 0.99 | Slight delay | Minor slippage; may self-correct |
| < 0.9 | Behind schedule | Significant delay; may impact deadline or cost |
Important SPI limitation: SPI is calculated in dollar terms (EV/PV), not in calendar days. At the end of a project, SPI always equals 1.0 (since total EV = total PV = BAC when complete) — regardless of how late the project finished. This makes SPI unreliable for measuring schedule delays near project completion. For time-based scheduling analysis, use Earned Schedule (ES) methods instead.
The 4 CPI/SPI Combination Scenarios
CPI > 1, SPI > 1
Under budget AND ahead of schedule. The ideal scenario. Investigate why — understand what's working so you can repeat it.
CPI > 1, SPI < 1
Under budget BUT behind schedule. Spending less than planned, but not completing work fast enough. May need to add resources to accelerate.
CPI < 1, SPI > 1
Over budget BUT ahead of schedule. Completing work faster, but at higher cost. Evaluate if the schedule gains justify the spending.
CPI < 1, SPI < 1
Over budget AND behind schedule. The worst case. Immediate management intervention required. Consider scope reduction or baseline revision.
Worked Example
A software development project has: BAC = $300,000, PV = $150,000, EV = $120,000, AC = $140,000
SPI = EV ÷ PV = 120,000 ÷ 150,000 = 0.800
This project is both over budget (CPI 0.857 = 16.7% cost overrun) and behind schedule (SPI 0.800 = 20% schedule slippage). The EAC factoring in both indices would be:
CPI vs SPI: Which One Matters More?
Both matter — but in different contexts:
- CPI is more predictive of final project outcome. Cost overruns compound: if you're inefficient now, the inefficiency continues. CPI is the primary metric for forecasting EAC.
- SPI matters most when deadlines are fixed. A fixed deadline contract or regulatory milestone makes SPI critical. A behind-schedule project may incur overtime costs that push CPI even lower.
- Use both together for the most complete picture. A project with CPI = 1.05 and SPI = 0.75 is doing well on cost but will likely miss its deadline.
Common Misconceptions
- "SPI > 1 means we're saving time." Not exactly — it means you're completing more budgeted work than planned. If your team is doing easier tasks first, this can be misleading.
- "CPI and SPI should always match." They frequently diverge. Projects that crash (add resources to accelerate) typically see SPI improve while CPI drops.
- "A CPI of 1.3 is always great." Extremely high CPI can indicate that scope was over-budgeted or that the team is cutting corners to appear efficient.
CPI interpretācijas ceļvedis
| CPI Vērtība | Statuss | Ko tas nozīmē |
|---|---|---|
| > 1.0 | Zem budžeta | Piegādā vairāk vērtības, nekā maksā katrs dolārs |
| = 1.0 | Budžetā | Piegādā tieši tā, kā plānots par dolāru |
| 0.9 – 0.99 | Neliels pārtēriņš | Rūpīgi uzraugiet; var būt nepieciešamas korektīvas darbības |
| < 0.9 | Virs budžeta | Būtiska izmaksu problēma; nepieciešama vadības rīcība |
SPI: Grafika veiktspējas indekss
SPI mēra, cik ātri darbs tiek veikts salīdzinājumā ar plānu. SPI 0.80 nozīmē, ka projekts norit tikai ar 80% no plānotā ātruma, kas nozīmē, ka projekts atpaliek no grafika par 20%.
SPI interpretācijas ceļvedis
| SPI Vērtība | Statuss | Ko tas nozīmē |
|---|---|---|
| > 1.0 | Apsteidz grafiku | Tiek paveikts vairāk darba, nekā plānots periodā |
| = 1.0 | Pēc grafika | Tieši atbilstoši plānotajam laika grafikam |
| 0.9 – 0.99 | Neliela kavēšanās | Neliela atpalikšana; var izlaboties pati |
| < 0.9 | Atpaliek no grafika | Būtiska kavēšanās; visticamāk būs nepieciešams paātrinājums vai virsstundas |
SPI ierobežojumi projekta beigās
Atšķirībā no CPI, tradicionālā EVM SPI ir trūkums: projekta beigās SPI vienmēr atgriezīsies pie 1.0, pat ja projekts kavējas gadu! Tas ir tāpēc, ka, pabeidzot visus darbus, EV vienmēr būs vienāds ar PV (kopējo plānoto vērtību). Lai risinātu šo problēmu, PM bieži izmanto Nopelnītā grafika (Earned Schedule) metodes.
Četras kombinācijas: CPI un SPI analīze
Vērot tikai vienu indeksu ir bīstami. Patiesais ieskats rodas, analizējot abus kopā.
- CPI > 1 un SPI > 1 (Sapnis): Jūsu projekts apsteidz grafiku un maksā mazāk nekā plānots. Priecājieties!
- CPI < 1 un SPI < 1 (Sliktākais scenārijs): Jūs atpaliekat no grafika un pārsniedzat budžetu. Šeit projekti izgāžas, ja vadība neiejaucas.
- CPI > 1 un SPI < 1 (Bieža problēma): Projekts kavējas, bet nauda tiek ietaupīta. Varbūt darbinieki nestrādā pietiekami ātri vai resursi ir novirzīti citur. Jums ir "jānopērk laiks", iztērējot ietaupīto naudu, lai paplašinātu komandu.
- CPI < 1 un SPI > 1 (Bīstams ātrums): Jūs strādājat ātrāk nekā plānots, bet iztērējat pārāk daudz naudas. Tas notiek, kad PM pievieno resursus, lai paātrinātu darbu (crashing), bet tas palielina izmaksas.