information systems analysis and design

Normalization: An Exercise

2009-05-03T04:22:00.000-07:00

Normalization Exercise

ERD:

First Normal Form (1NF)

On first normal form still have a lot of weakness, such us:
Insert, we can not only inserting NOPEG and NAPEG without their PROYEK, so we can inserting their attribute if there is PROYEK.
Update, depend on the table above, many attribute written recurring repeat, if there is updating attribute so all the same attribute must be update.
Delete, when one record vanished, so all data in that record will be vanished though the data still needed.
UMUR attribute must not be written, because every date of now at reducing attribute TGLLAH so will be result attribute UMUR.

Between Proyek entity and Bagian entity have one relation that is Punya relation, that meaning is one Proyek have many Bagian, so on Proyek table is filled NOBAG.

Between Bagian entity and Pegawai entity have two relation that is Anggota relation and Pimpin relation, that meaning is one Bagian have many Pegawai (Anggota relation) and one Pegawai leading one Bagian, so on Bagian table is filled NOPEG.

Pegawai entity have one relation that is Kontrol relation, that meaning is one Pegawai controlling many Pegawai (Unary relationship), that relation just using NOPEG.

On Tanggungan table is filled NOPRO and NOPEG because Tanggungan entity is weak entity, so Tanggungan entity dependency with Proyek entity and Pegawai entity.

On Kerja relationship have one attribute so the primary key from relation between Proyek entity (NOPRO) and Pegawai entity (NOPEG).

From the form above can be seen that 3NF form is equal with 2NF form because when it is created to 2NF form, a whole of attribute in table just depend from the only one attribute in that table. This is in accordance with the requirements of 3NF form that is in a table all of attributes must depend in an attribute that function as a primary key.

ER-Diagram

2009-04-19T05:57:00.000-07:00

Data Base Definition
A database is a structured collection of records or data that is stored in a computer system. The structure is achieved by organizing the data according to a database model. The model in most common use today is the relational model. Other models such as the hierarchical model and the network model use a more explicit representation of relationships.

Database Management System (DBMS)
A database management system (DBMS) is computer software that manages databases. DBMSes may use any of a variety of database models, such as the network model or relational model. In large systems, a DBMS allows users and other software to store and retrieve data in a structured way.

Bit

A bit is a binary digit, taking a value of either 0 or 1. Binary digits are a basic unit of information storage and communication in digital computing and digital information theory. Information theory also often uses the natural digit, called either a nit or a nat. Quantum computing also uses qubits, a single piece of information with a probability of being true.

Byte

byte is a basic unit of measurement of information storage in computer science. In many computer architectures it is a unit of memory addressing. There is no standard but a byte most often consists of eight bits.

Field
Field is a group of same byte, in database used by attribute.
Attribute
Attribute or Field is a characteristic from entity, which preparing detailed explanation about that's entity.

ATRIBUTE TYPES
Single Vs multi value
Single >> can only fill at most one value
Multi value >> can fill my more than one value with same type
Atomic Vs composition
Atomic >> indiscrete into smaller attribute
Composition >> representing alliance of some smaller attribute
Record
Record represent a data line in a relationship
Record consist of a corps of attributes which is interacted to give information about entity / relationship completely
Entity / File
File is a group of same record and have same element, same attribute but different of each data value.
In application process, file can be categorized as :
Main file
Transaction file
Report file
History file
Protector file
Activity file
DOMAIN
Domain represents corps of values which is enabled to stay in one or more attribute. Each attribute in a relational data bases defined as a domain
KEY OF DATA ELEMENT
Key is element of record which is used to find the record when accessing or also can used to identify each entity / record / Line
TYPES OF KEY
Super key represent one or more attribute from a tables which is able to be used to identify entity / record of tables uniquely ( not all of attribute can become super key)
Candidate Key is super key with minimum attribute. Candidate Key may not contain attribute of other tables so that candidate key beyond super key but of course not yet on contrary.
Primary Key is one of the attribute of candidate key can be selected /determined to become primary key with three criterions:
1.The Key more natural to be used as reference
2.The Key more simple
3.The unique of Key have to guaranteed.
Alternate Key is attribute of candidate key which is not chosen become primary key.
Foreign Key represent any attribute subjecting to primary key at other tables. Foreign Key will happened in relationship which is has cardinality one to many or many to many. Foreign Key usually is always put down at tables which is flange to many.
External Key represent a lexical attribute ( or gathering of lexical attribute) which is its values always identify one instance object.

ERD(ENTITY RELATIONAL DIAGRAM)

ERD is a model of a network that uses word order is stored in the abstract system.
The difference among DFD and ERD :
DFD is a model of network functions that will be implemented by the system
ERD is a model that emphasizes the network data on the structure and relationship data
Elements Of ERD
Entity
In the ER Diagram Entity is described with the form of a rectangle. Entity is something that exists in the real system and the abstract where the data stored or where there are data.
Relationship
A relationship is some association between entities. In this section we shall concentrate on binary relationships. That is, associations between two entities. In section 16.4 we shall introduce other N-ary relationships. That is, relationships between one, three, four or N entities. In the E–R approach, more than one relationship can exist between any two entities.
Relationship Degree
Are total entity who participated in one relationship. Degree that frequent being used on ERD
Attribute
Are character or characteristic of every entity and also relationship.
Cardinality
Pointing out record optimum that relations can with entity on entity another one.
CARDINALITY
There are 3 relationship cardinality that is
One to One :Level of Relation one to one expressed with one occurrence at first entity, only having one relation with one occurrence at second entity.
One To Many or Many to One :Level of Relation one to many is equal to many to one .For one occurrence at first entity can have many relation with occurrence at second entity, if on the contrary one occurrence at second entity only can has relation with one occurrence at first entities.
Many To Many : happened if each occurrence at an entity has many relation with occurrence at other entity

ERD(ENTITY RELATIONAL DIAGRAM)

In software engineering, an Entity-Relationship Model (ERM) is an abstract and conceptual representation of data. Entity-relationship modeling is a database modeling method, used to produce a type of conceptual schema or semantic data model of a system, often a relational database, and its requirements in a top-down fashion.
Diagrams created using this process are called entity-relationship diagrams, or ER diagrams or ERDs for short.

DFD quiz

2009-04-09T20:51:00.000-07:00

Data Flow Diagram
Figuring system division into smaller module
Facilitate user that insufficiently understand about computer area to understands system that will be worked
Context Diagram
- Consist of one process and figures scope of a system.
- Constituting supreme level from DFD that figure all of input and output of system.
- System limited by boundary (figured by dotted line)
- Stored is may not there

Zero Diagram
Zero diagram describes the process of the DFD. It's provides a view of the overall system, showing the main function or process that is, and the showing the flow of data and external entity. At this level, the data storage is possible. The process is no longer detailed on the next level then added the symbol '*' or 'P' at the end of the process. The input and output balance between zero diagram and context diagram should be maintained.

Detailed Diagram
Detailed Diagram is diagrams that decompose a process that is in the diagram zero level or above
Numbering level in the DFD.
In one level there should be no more than 7 units and the maximum of 9, when more should be done in the decomposition

1. External Entity
Each system has certain boundaries that separate a system with the environment outside the system. The system will receive input and produce output to the outside environment. External entity is an entity in environment outside the system that can be a human being, organization, or the other system. Most external entity is one of the following:
1. An office, department or division in the company but outside the system that is being developed.
2. Person or a group of people in the organization but outside of that system is being developed.
3. An organization or a person outside the organization such as customer and supplier.
4. Other information systems outside the system that is being developed.
5. Original source of a transaction.
6. Recipient of a final report produced by the system.

2. Data Flow
Data flow is a place of information flow. Data flow depicted with a straight line connecting the components of the system. Data flow is shown with the direction arrows and the name on the flow of data that flows. The data flow is pour between processes, storage data, and its indicates that the form of data input for the system.
Here are some examples of data flow:
a. Forms or documents used in the company.
b. Printed reports generated by the system.
c. Display or output on a computer screen generated by the system.
d. Input to the computer.
e. Speech communication.
f. Letter or memo.
g. Data is read or recorded to a file.
h. A field note book on the agenda.
i. Transmitting data from one computer to another.
Flow data should be given the name and have a clear meaning. The name of the data flow in addition to the written its arrow.

Guidelines for naming:
1. The flow of data flow consists of several words associated with the line speed
2. No data flow of the same name and should reflect the contents
3. Flow of data that consists of several elements can be expressed with the group element
4. Avoid using the word 'data' and 'information' to give a name to the flow of data
5. Wherever possible the complete flow of data is writte

3. Process
A process is an activity or work done by the person, machine or a computer from the result of data flow into the process for generated data that will flow out of the process. The process is what is done by the system and then can process data flows or data entry into the flow of data out. The transform function of one or more of data input into one or more of the output data in accordance with the desired specifications so each process has one or more inputs and produce one or more output. The process is also called bubble.
A process can be indicated with a circle symbol or rectangle with straight corners blunt.

Each process must be given a full explanation include the following.
1. Identification process. This included the identification of a number that indicates the number of reference and the process is written at the top of the symbol in the process.
2. Name of the process. The name indicates that the process is done by the process. Name of the process must be clear and complete.
3. Processor. The processor may not be listed.

Guidelines naming of the process:
1. The process consists of a verb and noun, which reflects the function of the process.
2. Do not use the process as part of the name of a bubble.
3. There may be some process that has the same name.
4. The process must be given a number. Order number wherever possible to follow the flow of the process or sequence, but the sequence number does not mean that the absolute is a process in chronological order.

4. Data Store
Data storage is a storage place for data that exists in the system.
Symbolized with a pair of parallel lines or two lines with one side open side. The process can retrieve data from or provide data to the database.
Guidelines for naming:
1. The name should reflect the data storage.
2. When its name more than one word must be marked with the number.

Data store can be as follows:
a. A file or database in the computer system.
b. An archive or records manual.
c. A box of data.
d. A table reference manual.
e. Agenda or a book.

Type of Data Flow Diagram
There are 2 forms of DFD, the physical data flow diagram and logical data flow diagram. Physical data flow diagram more emphasis on how the process of the system is applied while
logical data flow diagram is to emphasize the process of what is found in the system.
1. Physical Data Flow Diagram, more appropriately used to describe the existing system. The emphasis on this diagram is how the processes of the system is applied. PDFP must include the following:
1. Manual processes are also described.
2. Name of the flow data must indicate the number of such application form and the medium.
3. Savings data can show savings of non-computer.
4. The name of saving the data type of application must show, whether manual or computerized.
5. The process must show the name of the processor, the man, ministry, computer system or computer program to access the process.
2. Logical Data Flow Diagram, more appropriately used to describe the system that will be proposed. LDFD does not emphasize on how the system implemented, but the emphasis only on the logic of the needs of the system.
For computerized systems, drawing LDFD that shows only the needs of the proposed logic of system, usually the processes described is only the computer proces only.

Data Dictionary
Data dictionary is to help the system to interpret the application in detail and organize all elements of the data used in the system precisely so that the system analyst have a basic understanding of the same input, output, storage and process.
At analysis step, the data dictionary is used as a means of communication between the systems analyst with the user, and at the design stage of the system, the data dictionary is used to design input, reports and databases. Load the data dictionary as follows:
1. The flow of data: should be noted that readers who need further explanation about a flow of data can find it easily.
2. Alias: alias or other name of the data can be written when it is existence.
3. The form of data: used to segment the data dictionary to use when designing the system.
4. Data flow: indicates from which data flows and where the data goes.
5. Description: provides an explanation about the meaning of the data flow.

Balancing in DFD
Balancing in DFD means flow of data into and out of a process must be the same as the flow of data into and out of the details of the process on the level/levels below. Besides then the flow of data into and out of the process must match the name of the flow of data into and out of the details of the process. The number and name of an entity outside the process must be equal to the number of names and entities outside of the details of the process.
There are some issues that must be considered in the DFD which have more than one level:
1. Must be balance between input and output of one level and next level.
2. Balance between level 0 and level 1 at the input / output of stream data to or from the terminal on level 0, while the balance between level 1 and level 2 is seen on the input / output of stream data to / from the process concerned.
3. The flow of data, the data storage and the terminals at each level must be the same if its the same object.

Prohibition in DFD
1. Flow data may not be from outside the entity directly to other outside entities without going through a process.
2. Flow data may not be from the savings directly to the data to outside entities without going through a process.
3. Flow data may not be saving the data directly from the other savings data without going through a process.
4. Flow of data from one process directly to the other without going through the process of saving data should or be avoided as much as possible.

fourth quiz

2009-03-29T04:56:00.000-07:00

o CONCEPT OF INVESTIGATION SYSTEM
There are some point about Investigation System Concept, which the following:
1. Discrepancy between the objective of system and the real condition of system
2. Problem detection report is not real, because:
the objective is too ideal
lack of attitude and resources
system measuring is not accurate
the objective of system which have to be left behind
difference between the idle system and the temporary system
3. The Problem solving
investigation to the system which is running
getting the ideal system consensus
choose the best alternative
4. The objective of investigation have to show the problem which in fact is happened

o CONSTRAINTS ON INVENTIGATION SYSTEM :
When we doing the investigation, there are several constraints that are appear, such as:
1. Time
2. Cost
3. Knowledge
4. Politic
5. Involved
o RECOMMENDATION
The investigation results are a recommendation that include these following:
1. Never take any action if there is no problem was found
2. System maintenance some small problems
3. Increase ability or user ability
4. Totally consider to system modification
5. Put problem into the development system plan
o TACTICS OF INVESTIGATION
1. Listen the suggestion from system agent
2. Don't give the resolving early to problem
3. Compare stories.
4. Look for reluctant responses.
5. Avoid politics.

o INVESTIGATION TECHNIQUE

Directly:
1. The questionnaire
2. Monitoring
3. Observations
Not directly:
1. Flow of procedure
2. Reviewing documents
3. Sample
4. Tabular
o SYSTEM DESCRIPTION AT THE MOMENT
1. Input
2. Output
3. File
4. Data element
5. Transaction volume and action document
6. Data flow diagram
o ANALYSIS REQUIREMENT
Intensive interactions step between systems analyze and end user
where the team development system show their skill to get respons and user trust until achieve good participation.
There are 4 goal wishes to achieve:
1. Explain the system completely
2. Describe the ideal information system
3. Bring the ideal information system at moment with observe the resource constraint
4. Give push to user conviction into development system
Method:
1. Question and Answer
2. Questioner
3. Observation
4. Current procedure
5. Document monitoring
6. Resource problem
7. Time
8. Cost
9. Skill
10. Technology
11. External factor

o Document analysis requirement
1. Analysis guide: relationship with the end user, process monitoring, problems in data gathering.

2. User requirement: real needs, report needs, exercise needs, and new system influence.

3. System problem: explain time and cost constraint, skill, technology and external factor.

4. Document such as data gathering instrument, statistic consensus, data flow according to physic and logical, first data element in data dictionary.
o GENERATING SYSTEMS ALTERNATIVES
1. create alternative to solve the information system problem
2. Applying best alternative wisely.
o OPTIONS STRATEGIES
1. Distributed versus centralized processing
Changes in information decision-making from the centralized data processing to the decentralized end user responsibility center
2. Integrated versus dispersed database
System designer must consider the data which are entered in the database and into the file
3. Surround Strategy of System Development
About important environmental strategy in the case of company takeovers because the information system of other companies may vary with the company at this time.

o TACTIC OPTION
Operational planning selection
Planner selection is able categorize into:
1. Input
•Online Vs Off Line Data Entry
•Keyed Vs Machine Readable Data Entry
•Centralized Vs Decentralized Data Entry
2. Processing
•Batch Vs Real-time record update
•Sequential Vs Direct Access to records
•Single Vs Multiple User update of records
3. Output
•Traditional Vs Turn Around Documents
•Structured Vs Inquiry based reports
o SELECTING THE PROPER SYSTEM
1. Comparing tactics: System comparison based on the costs and benefits relatively; There are 3 ways a system is superior to the other, if:
- A's cost is lower than B and both are the same benefits.
- A's cost is lower than B and A yield more advantage than B.
- A and B have the same but the A's cost advantage more than B.
2. Some methods of comparison system
- Break Even point Analyisis
- Payback Period
- Discounted PayBack period
- Internal Rate of Return
3. Category Cost
- Hardware
- Software
- People
- Suppliers
- Teleccommunications
- Physical sites
4. Fee Details
Comparing the cost of information systems through the life of the system, analysis is calculating the cost changes for the future and there are 3 models of cost of information system, namely Linear, and exponentially Step Function.
5. Cost of information system can occur only once and can also occur continously.
- The cost of information systems that occur only once is ontime cost and development cost that occur during the development of the system.
- The cost of information systems that occur on a continual basis are reccuring cost and operational cost where the cost of this is happened when the information system operates every day.

o INFORMATION SYSTEM FACTORS
Qualitative factors that lead the performance of a good information system are:
Reduce errors
1. Reduce the time to fix errors
2. Reducing the response time from the alternative workstation
3. Accelerating the provision of information
4. Increase the security system
5. Have active update source record
6. Increase user satisfaction

o CORPORATE STRATEGY FACTORS
1. Customer satisfaction
2. Level sales increased
3. Commitment of consumers and vendors
4. Information of marketing products

o PRESENTING LEARNING OF SYSTEM

1. Conduct a brief presentation
2. Reduce technical explanation in detail
3. Presenting clearly with visual aids
4. If using a model using a tool such as a laptop so that more informative
5. Emphasize the benefits of the proposed information system with some alternative that is appropriate conditions experienced by the company.

o CONTINUING THE DECISION OR NOT
1. If the company decided to develop the system, the information department will do next, namely the System Design Process.
2. If the other way then the System Development Life Cycle (SDLC) will be terminated.
3. Often a problem will be found with the study system and then usually top management will ask for the re-employment the study system.
4. Model will describe some of the stages of the repeated and sometimes the information will make a decision before the repeat stage system of study.
5. With the alternative, the decision to repeat a previous stage SDLC or not is called Go-No-Go Decision.

third quiz

2009-03-13T20:19:00.000-07:00

The Importance of System Development

Problem – an undesirable situation that prevents the organization from fully achieving its purpose, goals, and/or objectives
Opportunities
Increase the equipping of information until the information can used to make a decision. The opportunities are like: market opportunity, costumer services.
Directive - a new requirement that is imposed by management, government, or some

external influence.
The Purpose of Developing an Information System

After developing new system, expected improvement in new system. This improvement relate to PIECES, that is:
• Performance, improvement of new system performance so that more efficient. Performance can be measured from throughput and response time. Throughput is the amount of work which can be done certain in a moment. Response Time is average time which delays among two work added with response time to answer the work.
• Information, improvement of quality of presented information.
• Economic, improvement of advantage or benefit.
• Control, improvement of controlling for detecting and repairing some mistake and also insincerity.
• Efficiency, improvement of operating efficiency.
• Service, improvement of service given by system.

The principles of developing an information system
1. System is developing for management
2. System is the great investment
3. System need educated person
4. System development life cycle
5. System development process is must not in series
6. Don’t be afraid to canceled the project
7. System Documentation

System development life cycle• life cycle model waterfall a method system the working gradually, in every stage can not return previous step. The step composition is requirements, design, implementation, verification, maintenance. With waterfall method, if step be verification we can’t previous to step design.
• life cycle with method itratif, by using this method our first does with initial planning, then requirement, analysis and design, then implementation, deployment, besides deployment we also do testing, then continued with evaluation. if there deficit or error in system, we can do new planning. this be difference between method waterfall with itratif. itratif can do change if there error in system.
• from life cycle spiral model, first step at does in spiral method concept of requirement that present in stage development, then concept of operation this present in stage plan, stage requirement plan that present in stage determine, risk analysis this present in stage identfy and resolve risk, get a temporary result then that is continued with stage venfication and validation, then development plan, risk analysis, produce a prototype 2, then at continue with venfication and validation, then at continue with test plan, continued again with risk analysis and produce oprational prototype, continued with detailed design, code, integration, test, implementation, and last is done release from system made.

approaches of developing a system
• will approach classic opponent will approach structured will approach this will emphasized that system development success when follow stage at system life cycle.
- pendekatan classic approach in system development that follow stages at system life cycle without supplied with tools and techniques memadai.
- pendekatan structured approach this that begun from beginning year 1970 called with approach structured. approach structured equiped with tools and techniques that wanted in system development so that end result from system that developed be got system so that end result from system the structure will be be defined well and clear.
• approach pieces opponent approaches system.
approach pieces be to approach system development that emphasized at one particular activity or certain application. approach system that pay attention information system as one integration unitary to each activity or the application.
• approach system comprehensive opponent approaches
approach sistem-menyeluruh be to approach menyang system together according to comprehensive. will approach this less synchronized for system complex because difficult to be developed. this be to approach classic approach modulate try broken complex system become some part or simple module, so that system easier will be understood and developed.

• approach opponent far jump approaches to bloom.
approach to jump -jauh apply change according to together use sophisticated technology. this change is many contains risk, because computer technology so fast bloom and to year coming be worn out. approach this too expensive, because need investment at the same moment. approach to bloom to apply sophisticated technology just for applications that need at that moment and then be developed for periods next will follow the need as according to existing technology development

meaning of methodology, method and algorithm
• methodology is unitary from methods, procedures, job concepts, rules that used by a science, art, or discipline the other
• method is a steps to in methodology
• algorithm is sequence procedure to break problem.

three classifications of development methodology

• functional decomposition methodologies
this methodology constitute in solution from system into sub system smaller. belong into group methodologies this:
- HIPO (hierarchy plus-process-output)
- Stepwise refinement (SR) or iterative stepwise refinement(ISR)
- Information-hiding
• Data oriented methodologies
- Data flow methodologies
this methodology is in general is based in solution from system intoes modules based on from type element data and module logic behaviour in system
- Data structure oriented methodologies
this methodology emphasizeds structure from input and output at system. mask in this methodology: jsd (jakson's systems development) and w/o(warnier/orr)

tools for developing a system
• system development tool:
HIPO diagram, used in metodelogi HIPO
Data flow diagram, used in system analysis and design S
Structured chart, used at method wanier/orr
Jakson's diagram, used at methodology jakson's system development

Techniques used in developing a system
techniques that can be used :
• technique management project, that is CPM(critical part method) and PERT (program evaluation and review technique). This technique used in project scheduling.
• Fact finding techniques, that is technique that can be used to gather fact and find data in activity study existing activity.
• technique analysis cost/benefit (cost-effectiveness analisys or cost-benefit analysis)
• technique to run meeting
• technique inspection/walkthrough

system analyst and a programmer
• system analysis one who analysis system (study that problems and determine system user needs) identify solution that have occasion
• a programming one who write program codes into computer system. where is code at write by virtue of analysis from a system analysis.

mention what are the knowledge a system analyst should posses
• knowledge and skill about technique processing data
• knowledge about business in general
• knowledge about quantitative method
• trouble-shooting skill
• skill in communicate with staff
• skill in build connection with staff

resume 1

2009-02-21T00:07:00.000-08:00

Basic Concept of System
Before studying what that Information Systems Analysis and Design, first we have to know what that system and what that information.
What is system
There are two approaching to definition of system:
1.Representing an unity from some definition element or subsistem emphasizing at component or its element
2.Representing an procedure to reach the target of definition emphasizing its procedure.
Characteristic System
Characteristic a system can be explained as follows:
1. Components
Components which interact, what each other working along to form an unity
2. Boundary
Limiting system with environmental or other system outside its.
3. Environments
environment which always influence operaasi of system.
4. Input
Entered energy into system can in the form of treatment input and signal input. The treatment input is entered energy system to can walk. While signal input is energy which in process to yield output.
5. Output
Result from processed energy and classified is become an output which good for and rest of dismissal.
6. Process
Change an input become output.
7. Target/goal
Otherwise have the target then system operation there will be no utilizing of. System succeed if it have reach the target its goal.
8. strategy
To reach made cool to be target to be needed by an strategy to be the target can reach.
Classification System
1. Abstract System: system which in the form of idea or idea- idea which do not see physically.
2. Physical System: system visible and real system with eye, and every creature can see it ( Example: System Computer)
3. Natural System: system that happened through natural process when human don’t do anything. (Example: System of Galaxi).
4. Human Made System: system designed by human being (system machine), example: Information System.
5. Deterministic System: operating with behaviour which have earned estimated (example: System Computer).
6. Probabilistic System: system which condition of its future doesn’t earn estimated because containing probability element. (Example: Human System).
7. Close System: system which don’t correlate and don’t affect with external system of it. Theoretically the system is exist, but in reality there is no really system closed, existing only system closed relatively (relatively closed, do not really closed).
8. Open System: system which imbued and coresponding with external environmen.
Information
Information is processed data become more form good for and more meaning to accepting it.

Quality Of Imformation
To get information which with quality hence have to fulfill:
Accurately
Information free from mistakes and do not deflect or mislead
Punctually
Incoming information at receiver may not lose time
Relevantly
Information have benefit for its usage