Kinds of System and

Problem FramesLecture 6

Getting a Grip on Problems

• Are use cases enough?

• Two problems– The Mayday system

– The Volunteer system

• Common kinds of system– repository

– simulation

• Matching systems

• Review

Getting Started

• How do you start to tackle a new systems analysis/design problem?

• Use a Human-centred / organisationally-centred approach to problem identification

• Do detailed analysis - 100’s of use cases, sequence diagrams, class diagrams

• But what’s the big picture? What does the system DO?

Problem• An organisation has a mission to put volunteers with certain

skills, interests and available time in touch with community groups or individuals with specific needs.

• Assignments last for a period of time ranging from a day to a year. Volunteers can often take several assignments at a time, and groups may need several helpers. The organisation tries to achieve a good match between volunteers and groups, resulting in valued, lasting relationships

• Currently assignments arrive at the rate of 20 a day, and new volunteers arrive at the rate of 5 a day.

What kind of System is it?• system is a repository

• system is a simulation of the ‘real world’– system is a resource manager – system is a match-maker

• system is a controller of real world activity

• system is a co-ordinator of human activity

• system is a tool for construction of an artifact

• system is a learning system

• .....

• Real systems have aspects of several kinds but it helps to focus of each viewpoint in turn

System is a Repository

• We can view many systems as simple repositories of data.• Data is collected, stored, kept safe from falling into the wrong

hands, from being lost.• Data can be extracted, reorganised, removed if no longer

relevant• Data should only be accepted if it conforms to rules for good

data -‘integrity rules’• Simple-minded Database view of systems - the CRUD view.

Repository

• In the problem, we need to be able to store data about volunteers and clients

• To be able to add new data at a rate to keep up with arrival of new volunteers and clients

• Need to address such issues as how to keep the data secure, available, safe

• The Scope of this view is the Computer System• Metaphor : SYSTEM IS A BANK

System is a Simulation • System simulates some aspects of the real world • Design problems:

– how much of the real world to model, in what detail to support needs of users, now and in the future?

– how to ensure that the model and the real world stay closely in synchrony - that changes to the real world (a volunteer moves house) are reflected in the model (address field updated) quickly and accurately

– how to detect changes in the real world and communicate these to the system

– how and what derived information is needed

Simulation

• The scope of this view is the whole Information System

• Jackson calls this the ‘Information Display’ Frame

• Metaphor: SYSTEM IS A MAP

• System is a Simulation has many subtypes:– system is a resource manager– system is a match maker– system is a classifier– ....

System is a Resource Manager

• Allocation of scarce resources, such as – seats on flights– rooms for lectures– beds for patients

• Problem is to maximise utilisation of resources whilst minimising delay and loss of business

• A common sub-problem is to match a client’s needs to available resources

System is a Match-Maker• Many systems have a matching task at their core.

– fingerprint matching– patient/donor matching for transplant surgery– patients to clinical trials– interns to placements in hospitals– parents with similarly challenged children– DNA samples– electronic devices - speakers to amplifiers– search request to relevant documents– incoming news items to information subscribers– finding a rescue boat– number plate recognition in London’s Congestion Congestion Charging

System

System is a Classifier

• Classification separates data into two or more classes– Filtering Email Spam– Classifying students by grade of degree– Classifying credit card transactions as fraudulent– Detecting spelling mistakes

• METAPHOR : SYSTEM IS A SIEVE

Classification ErrorsGood mail Spam

accept

reject correct

correct

false negative

false positive

The two errors are in conflict - we can decrease the risk ofa false positive (reject more Spam) but we increase the risk of false negatives (rejecting good email): a TRADE-OFF.

Matching in detail• Matching task typically involve:

– two sets of individuals : e.g.• the volunteers / available rescue craft - Resource• the community tasks / the distress vessel - Requirement

– ‘adequate’ representations of both – a ‘fitness’ function which calculates how well matched a

Requirement is to a Resource

• Different allocation constraints– single allocation - can only rescue one boat at a time– batch allocation - best solution for a set of volunteers– no allocation - searching the web

Single Allocation

• Allocation to a single Requirement:– ‘long list’ the Resources - eliminate the obviously

unsuitable (classification)– compute fitness between Requirement and each

remaining Resource– rank the Resources in fitness order for a ‘short list’– ? user selection from short list on basis of additional

information unknown to system

Creating a Fitness Function• Mayday

– can use minimum time to reach position, but a more distant boat may have better rescue, medical support, more fuel, hence need for judgement

• Community volunteers– travel distance between volunteer and group may be one factor

(complicated by roads and bus routes)– other criteria will be categorical - skills, likes and dislikes, age

• is electrician closer to mechanic than builder?

– hard to combine into a single, meaningful measure of fitness - combining travel distance and skill match

Fuzzy matching

• How close are two ages - e.g. in a dating agency, age of applicant, desired age of partner?

min max0.0

1.0

age

?

Combining fuzzy values

• Multiple qualities(dimensions) need to be compared but how?

• In the Mayday system, we combine two dimensions (Lat and Long) using a variant of Pythagoras (X2+Y2

= R2 )

• In the Volunteer system:– put weights (reflecting importance) on each dimension

( age, travel, skill..)– compute sum of squares of weighted differences

System is a learning system• To define the Fitness criteria

– need to carefully research the criteria for a good match, but not likely to get measures, weights right first time

• Good fitness functions need to be learnt– collect evidence of the quality of the resultant match in order

to refine matching criteria, to detect missing criteria, to determine the weights to be given to separate criteria

• Learning affects the total system – of data collected, of recruitment of volunteers, as well as

amendment of the software

A Learning system• Learning cycle:

– action - put current rules into practice– observation - record results of rules– analysis - analyse reasons for any differences– change - modify the rules to improve performance

• Feedback loop, comparing goals with actual performance

action

observation

analysis

change

Framework Approach to Analysis

• Identify core task areas

• Apply common framework to specific case

• Framework provides:– standard terminology– checklist of issues which need to be addressed– index of solutions to commonly occurring

problems

Commercial Of the Shelf Software (COTS)

• Software exists for most business needs:– payroll– order processing– general ledger– human resources– e-commerce– e.g. SAP, SAGE ..

• but analysts need to match business needs to COTS capability, and customise generic software for local business rules.

Related Material

• Michael Jackson (2001) Problem Frames, Addison-Wesley takes a more formal approach.

Tutorial

• Choose a couple of different matching problems, or identify your own.

• Research the fitness function which would be appropriate for these problems

• Identify the processes required to implement a learning system around each matching system