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Better Specifications. What is a Specification?. A Statement of the Customer’s Needs In the Form of Required Characteristics of a Product A Component of a Legal Contract Between Customer and Supplier Subject to Verification of Compliance. Specification Organization. MIL-STD 490

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What is a specification l.jpg
What is a Specification?

  • A Statement of the Customer’s Needs

  • In the Form of Required Characteristics of a Product

  • A Component of a Legal Contract Between Customer and Supplier

  • Subject to Verification of Compliance

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Specification Organization

  • MIL-STD 490

  • Should Follow Functional Decomposition

  • PUI Organization can Help

  • Performance Requirements Not Decomposed Further When Can Allocate to Next Level of Hierarchy

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Ground Rules

  • Say What You Mean Precisely and Simply

  • Only “SHALL” Imposes a Contractual Requirement

  • Do Not Use Subjective Adjectives (large, rapid, modular, nominal,optimum, efficient)

  • Define Verification Criteria as part of Requirement Statement Generation

  • Provide Definitions

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Some Characteristics of Good Requirements

  • Clear

  • Necessary

  • Applicable to only One System Function

  • Not Redundant

  • No Conflict With Other Requirements

  • Not Biased by any Particular Implementation

  • Verifiable

  • Traceable

  • Achievable

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Partial List of “Poor Words”

  • Adequate – Not verifiable

  • And – Possible multiple requirements

  • Appropriate – Not Verifiable

  • Best Practice – Not verifiable

  • But not limited to – Unspecified super set, not verifiable

  • Easy – Not verifiable

  • For Example – Not verifiable

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As a minimum




User Friendly




Provide for


More Not Verifiable “Poor Words”

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Parent Child RelationshipsSpecification View

3.2.x Power-up Missile

3.2.x.1 Receive External Power

3.2.x.2 Perform SBIT

3.2.x.2.y Computer SBIT



3.2.x.3 Go to Standby

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Some Problems

  • Specifying Capability – Does not Ensure Delivered Unit Will Perform as Desired.

    • “…the unit’s design shall be capable …”

  • Design Could be Correct but Unit Improperly Fabricated.

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Problems (Continued)

  • Absolute Performance Parameter Requirements

    • Variability Exists (Profound Knowledge)

    • Tolerance Range Required

    • Values, not Percentages

    • Define Origin for Time Requirements

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Problems (Continued)

  • Use of “Support”

    • “…The XXX shall support …”

  • How do You Verify Compliance?

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A Well Written Specification is Not:

  • A Design Description

  • A Statement of Work

  • A Test Plan

  • A Concept of Operations Description

  • A Novel

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Electric Water Heater Controller

  • What is the Required Function?

  • Output = 3000 W; 70°<T<100°

  • Output = 2000 W; 100°<T<130°

  • Output = 1000 W; 120°<T<150°

  • Output = 0; 150°<T

  • Is this Requirement Set Valid?

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Requirement Examples

  • Examples:

    • Good requirement statement:

      “The system shall provide a water flow rate of 500 gallons per minute ±10 gallons per minute.”

    • Lousy requirement statement:

      “The system should provide a flow rate to the maximum extent possible.”

Courtesy of Reed Integration, Inc.

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IBIT Response Time

The XXX shall complete IBIT and transmit the test results to the MC within 5 minutes.

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CBIT Coverage

The XXX design shall allow CBIT to test and status functions on a non-interfering

continuous basis.

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IMU Operating Time

The IMU shall operate 10 minutes for tactical operation.

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Acceleration Measurement Range

The IMU Accelerometer provides velocity measurement data along the three orthogonal axes

for accelerations not exceeding 50g along any axis.

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IBIT Coverage

The XXX design shall allow IBIT to test no less than 99 (TBR) percent of the electronic function

critical missile faults. The following minimum functions shall be tested:

Transmission of software version in Read Only Memory to MC.

Determine internal temperature of package

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Examples:Good Requirements Statements

  • The electrical power subsystem shall provide 28 +4, -2 vdc to the system.

  • The weight of individual items shall comply with MIL-STD-1472 paragraphxxx.yyy.1 titled “Single person lift requirements”.

  • The command generator process shall receive inputs from the operator, and generate commands in the format specified in IFS 10345.

  • The alignment error caused by structural deformation due to loads and thermal gradients shall be no greater than 0.01 deg.

  • The vehicle shall receive and authenticate commands from the ground station, and provide an acknowledgement that the command has been accepted

  • The vehicle shall perform as specified within the temperature range of –25 to +65 deg Celsius.

  • The system shall have the capability to accept no less than 25 targets.

Courtesy of Reed Integration, Inc.

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Some Famous Failures

  • Titanic – Rivet Quality Verification

  • Tacoma Narrows Bridge – Requirement not Valid

  • Apollo 13 – Failure to Change Spec and Verify Compliance

  • IBM PC Jr – Requirement not Valid

  • See Bahill & Henderson, Systems Engineering, Vol 8, Nr 1, 2005, pp 1-14

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  • Most Specs are Invalid and not Verifiable

  • Poor Specs lead to Program Problems

  • Product Assurance Begins with Ensuring Valid and Verifiable Specifications

  • If it is not in the contract it will not be delivered.

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Compliance Verification

  • Operational Definitions

  • Compliance Verification Criteria

    • Source

    • Where Documented

  • Integration Into Program

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Operational Definition

  • A definition by which one can do business

  • Without benefit of lawyers

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  • Need – A blanket, 50% wool, 50 % cotton

  • Requirement Statement

    • “The blanket shall contain a 50/50 blend of wool and cotton fibers.”

  • Possible Solution

    • Take a wool blanket and a cotton blanket

    • Cut each in half

    • Sew a wool half to a cotton half

  • Question – Has requirement been satisfied? How do you know?

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Better Statement

  • “The blanket shall contain a homogeneous blend of wool and cotton fibers.”

  • Questions

    • How do we define homogeneous?

    • How do we define wool and cotton?

    • What method will we use?

    • What results are acceptable?

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  • Method

    • Pick 3 blankets at random from a lot of 200

    • For each blanket pick 10 locations randomly distributed across the surface of the blanket.

    • Cut a one inch diameter circle sample at each location using a ruler to measure diameter.

    • Subject each sample to fiber classification in accordance with XYZ standard.

    • Measure the weight of each fiber type using a calibrated laboratory scale.

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Solution (Continued)

  • Criteria

    • If the proportion by weight of wool to cotton or cotton to wool is at least 45/55 for all samples, accept the blend as homogeneous.

  • We can do business!

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  • Every specification requirement statement containing SHALL must have an associated criterion for verifying compliance.

  • No requirement statement is complete until the verification criteria are defined

  • If you can’t figure out how to verify compliance, it should not be a requirement.

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Verification Methods

  • Inspection

  • Demonstration

  • Analysis

  • Test

  • Method identified in Verification Cross Reference Matrix

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Verification by Inspection

  • Physical Properties

    • Weight

    • Center of Gravity

    • Moments of Inertia

  • Dimensions

  • Appearance

  • Will have tolerance or standard

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Verification by Demonstration

  • Maintenance times

  • Speed

  • Load

  • Transportation

  • Environmental qualification

  • Go/No go, (Bernoulli Process)

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Verification by Analysis

  • Kill probability

  • Performance range

  • Effectiveness

  • Efficiency

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Verification by Test

  • Accuracy

  • Compatibility/interactions

  • Reliability

  • Timing

  • Functioning

  • Mechanical Properties

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Problem Areas

  • Stochastic parameters

    • Reliability

    • Accuracy (variability)

  • Origin definitions

    • From where do we begin measuring

  • Event definitions

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Accuracy Compliance

Requirement:Sigma < Sigma0

Use hypothesis test

Alpha = 0.05

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Hypothesis Testfor Accuracy ComplianceHo: Sigma=Sigma0H1:Sigma < Sigma0

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Operational Characteristic

  • P[Accept Ho| False] vs. Sigma|Sigma0

  • β vs Sigma|Sigma0

  • Depends on Sample Size

  • Depends on α

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Risk of not satisfying the requirement is high unless:

  • Demonstrated Accuracy is much better than actually required


  • A very large sample is obtained with accuracy close to the requirement.

    In any case, Demonstrated Accuracy Must be better than the Requirement

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Better Way

Specify a tolerance around a target point and a minimum number of samples.

Compliance verification criterion is clear.

Note: Will still have to evaluate risk of unacceptable performance as a function of tolerance and sample size.

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Origin and Definition Problems

  • Requirement:

    • “The XYZ shall activate within 10 milliseconds after receipt of an activation command”

  • Problems:

    • What event corresponds to “activate?”

    • What is the definition of “receipt?”

    • When does one start and stop measuring?

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  • Section 4, Quality Assurance Provisions, should contain the compliance verification criteria.

  • The verification cross reference matrix should should identify the verification method and reference the paragraph that contains the criteria

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Verification Example

Specification Statement

3.1 Blanket Material Properties – The blanket fabric shall

be a homogeneous blend of 50% wool and 50% cotton

Verification Cross Reference Matrix

Para Title Method Procedure

3.1 Blanket Material Properties T Para 4.3.1

. . . .

. . . .

. . . .

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Program Implementation

  • Since every requirement containing “shall” is a contractual requirement, compliance must be verified.

  • A procedure for collection, documenting, and displaying compliance information should be defined and managed. Requirements Allocation Database is a Possibility.

  • Criteria should be developed in conjunction with the requirement.

  • A master plan (Verification Plan) to integrate compliance verification activities into development tests and other activities can be developed, but criteria must be provided to the planners.

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  • Requirement compliance verification requires operational definitions.

  • Requirements are not complete without compliance criteria.

  • Compliance verification can be easy or hard, depending on how the requirement is stated.