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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 • 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 • Monitor • Large • Sufficient • Many • User Friendly • Maximize • Quick • Minimize • Effective • As a minimum • Normal • Rapid • Provide for • Should Parent Child Relationships Specification 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? 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. Example IBIT Response Time The XXX shall complete IBIT and transmit the test results to the MC within 5 minutes. Example CBIT Coverage The XXX design shall allow CBIT to test and status functions on a non-interfering continuous basis. Example IMU Operating Time The IMU shall operate 10 minutes for tactical operation. Example Acceleration Measurement Range The IMU Accelerometer provides velocity measurement data along the three orthogonal axes for accelerations not exceeding 50g along any axis. Example 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 Examples: Good Requirements Statements 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 paragraph xxx.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. 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 Test for 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 1 0.9 0.8 0.7 0.6 P[Accept Ho] n=3 0.5 n=20 0.4 0.3 0.2 0.1 0 0 0.2 0.4 0.6 0.8 1 1.2 Sigma/Sigma 0 CEP Confidence Limit vs Sample size 2.0000 CEP Upper Limit/CEP 1.8000 C=0.95 Estimate 1.6000 C=0.9 1.4000 C=0.85 C=0.8 1.2000 1.0000 0 5 10 15 20 25 30 Sample Size Problem Risk of not satisfying the requirement is high unless: – Demonstrated Accuracy is much better than actually required Or – 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 Procedure 3.1 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 188.8.131.52 Select three blankets at random from each lot in accordance with paragraph 4.x.y (randomization procedure) 184.108.40.206 For each blanket, select ten locations randomly distributed across the surface of the blanket in accordance with paragraph 4.x.z. 220.127.116.11 Cut a 1.0 0.1 inch diameter circular sample from the blanket at each of the selected locations. 18.104.22.168 Subject each sample to fiber classification by weight in accordance with Procedure XYZ. 22.214.171.124 Declare the sample compliant if the classification result is at least 45%/50% wool to cotton OR cotton to wool. 126.96.36.199 Declare a blanket compliant if at least nine of the ten samples are compliant. 188.8.131.52 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.
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