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

• Should Follow Functional Decomposition

• PUI Organization can Help

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

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

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

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

More Not Verifiable “Poor Words”

• Including • Large • Many• Maximize• Minimize• As a minimum• Rapid

• Monitor• Sufficient• User Friendly• Quick• Effective• Normal• Provide for• Should

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

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.

Problems (Continued)

• Absolute Performance Parameter Requirements– Variability Exists (Profound Knowledge)– Tolerance Range Required– Values, not Percentages– Define Origin for Time Requirements

Problems (Continued)

• Use of “Support”– “…The XXX shall support …”

• How do You Verify Compliance?

A Well Written Specification is Not:

• A Design Description

• A Statement of Work

• A Test Plan

• A Concept of Operations Description

• A Novel

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?

•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.”

Requirement Examples

Courtesy of Reed Integration, Inc.

Example

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

Example

CBIT CoverageThe XXX design shall allow CBIT to test and status functions on a non-interferingcontinuous basis.

Example

IMU Operating TimeThe IMU shall operate 10 minutes for tactical operation.

Example

Acceleration Measurement RangeThe IMU Accelerometer provides velocity measurement data along the three orthogonal axesfor accelerations not exceeding 50g along any axis.

Example

IBIT CoverageThe XXX design shall allow IBIT to test no less than 99 (TBR) percent of the electronic functioncritical missile faults. The following minimum functions shall be tested:

Transmission of software version in Read Only Memory to MC.Determine internal temperature of package

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

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

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

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

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

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

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

Examples:Good Requirements Statements

Courtesy of Reed Integration, Inc.

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

Observation

• 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.

Compliance Verification

• Operational Definitions

• Compliance Verification Criteria– Source– Where Documented

• Integration Into Program

Operational Definition

• A definition by which one can do business

• Without benefit of lawyers

Example

• 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?

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?

Solution

• 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.

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!

Applicability

• 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.

Verification Methods

• Inspection

• Demonstration

• Analysis

• Test

• Method identified in Verification Cross Reference Matrix

Verification by Inspection

• Physical Properties– Weight– Center of Gravity– Moments of Inertia

• Dimensions

• Appearance

• Will have tolerance or standard

Verification by Demonstration

• Maintenance times

• Speed

• Load

• Transportation

• Environmental qualification

• Go/No go, (Bernoulli Process)

Verification by Analysis

• Kill probability

• Performance range

• Effectiveness

• Efficiency

Verification by Test

• Accuracy

• Compatibility/interactions

• Reliability

• Timing

• Functioning

• Mechanical Properties

Problem Areas

• Stochastic parameters– Reliability– Accuracy (variability)

• Origin definitions– From where do we begin measuring

• Event definitions

Accuracy Compliance

Requirement:Sigma < Sigma0

Use hypothesis test

Alpha = 0.05

Hypothesis Testfor Accuracy Compliance

Ho: Sigma=Sigma0

H1:Sigma < Sigma0

Action

State Accept Ho Reject Ho

Ho True No Error Type 1 Error

Ho False Type 2 error No Error

Operational Characteristic

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

• β vs Sigma|Sigma0

• Depends on Sample Size

• Depends on α

OC Curves. Alpha = .05

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1 1.2

Sigma/Sigma 0

P[A

cce

pt

Ho

]

n=3

n=20

CEP Confidence Limit vs Sample size

1.0000

1.2000

1.4000

1.6000

1.8000

2.0000

0 5 10 15 20 25 30

Sample Size

CE

P U

pp

er L

imit

/CE

P

Est

imat

e

C=0.95

C=0.9

C=0.85

C=0.8

Problem

Risk of not satisfying the requirement is high unless:– Demonstrated Accuracy is much better than

actually requiredOr– A very large sample is obtained with accuracy

close to the requirement.In any case, Demonstrated Accuracy Must be

better than the Requirement

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.

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?

Documentation

• 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

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 Procedure3.1 Blanket Material Properties T Para 4.3.1 . . . . . . . . . . . .

Verification Example (Cont’d)Section 4.0 4.1 Inspection Procedures 4.2 Demonstration Procedures 4.3 Test Procedures 4.3.1Blanket Material Properties 4.3.1.1 Select three blankets at random from each lot in accordance with

paragraph 4.x.y (randomization procedure) 4.3.1.2 For each blanket, select ten locations randomly distributed across the

surface of the blanket in accordance with paragraph 4.x.z. 4.3.1.3 Cut a 1.0 0.1 inch diameter circular sample from the blanket at each

of the selected locations. 4.3.1.4 Subject each sample to fiber classification by weight in accordance

with Procedure XYZ. 4.3.1.5 Declare the sample compliant if the classification result is at least

45%/50% wool to cotton OR cotton to wool. 4.3.1.6 Declare a blanket compliant if at least nine of the ten samples are

compliant. 4.3.1.7 Declare the lot compliant if all sampled blankets are compliant.

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.

Summary

• 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.