CPI and SPI Query

Earned Value Management

SASWATA MANDAL Management KOLKATA, WB, India

Hi Experts ,

Lets say , a project has CPI = .9 and SPI = .9 .

So, CPI < 1 ie. the project is Over Budget and SPI < 1 ie. the project is Behind Schedule .

CPI = .9 ie. ( 1 - .9 ) = .1 ie. ( 1 / 10 ) x 100 = 10 % . So, the project is 10% Over Budget.

SPI = .9 ie. ( 1 - .9 ) = .1 ie. ( 1 / 10 ) x 100 = 10 % . So, the project is 10% Behind Schedule.

I am unable to understand , why are we doing ( 1 - .9 ) ie. why I am deducting .9 from 1 ?

CPI = .9 , so , I could have said that , project is Over Budget by .9 ie. ( 9 / 10 ) x 100 = 90 %

SPI = .9 , so , I could have said that , project is Behind Schedule by .9 ie. ( 9 / 10 ) x 100 = 90 %

Why I am deducting .9 from 1 ?

Sorry , if this sounds a silly question !

Thanks & Regards

Saswata Mandal

Posted: Apr 3, 2026 2:52 AM

Imran Afzal Author| The Strategic PMO Cary, NC, United States

This is a great question—and not a silly one at all. The confusion usually comes from what CPI and SPI actually represent.

CPI and SPI are not percentages.

They are efficiency ratios.

CPI tells you: how much value you’re getting for every $1 spent.

SPI tells you: how much progress you’re making for every unit of planned time.

So if CPI = 0.9, it means:

For every $1 you planned to spend, you’re only getting $0.90 of value.

That’s why we don’t say “90% over budget”—that would be a completely different meaning.

To express the variance, we compare against the baseline of 1.0 (which represents “on plan”).

So:

1.0 − 0.9 = 0.1 → meaning you’re 10% less efficient than planned

That’s where the “10% over budget” interpretation comes from.

Same idea for SPI:

SPI = 0.9 means you’re progressing at 90% of the planned rate

So:

1.0 − 0.9 = 0.1 → you’re 10% behind schedule

The key insight is this:

0.9 is not “90% over” anything—it’s “90% of expected performance”

And the gap from 1.0 tells you how far off you are.

If you said the project is “90% over budget,” that would imply something very different (almost double the cost), which is not what CPI = 0.9 means.

So subtracting from 1 isn’t arbitrary—it’s because 1.0 represents the planned baseline, and you’re measuring deviation from that baseline.

Once you think of CPI and SPI as performance efficiency ratios instead of percentages, the math and interpretation start to make a lot more sense.

Posted: Apr 3, 2026 2:37 PM

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CPI and SPI Query

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