Presentation on theme: "Better Specifications. What is a Specification? A Statement of the Customers Needs In the Form of Required Characteristics of a Product A Component of."— Presentation transcript:
What is a Specification? A Statement of the Customers 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 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 units 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°
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 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
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 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 cant 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 < Sigma 0 Use hypothesis test Alpha = 0.05
Hypothesis Test for Accuracy Compliance Ho: Sigma=Sigma 0 H1:Sigma < Sigma 0 Action StateAccept HoReject Ho Ho TrueNo ErrorType 1 Error Ho FalseType 2 errorNo Error
Operational Characteristic P[Accept Ho| False] vs. Sigma|Sigma 0 β vs Sigma|Sigma 0 Depends on Sample Size Depends on α
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 ParaTitle MethodProcedure 3.1Blanket Material Properties T Para
Verification Example (Contd)
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.