The Classification Model

Contact Martin Modell   Table of Contents

Throughout this book traditional systems design terminology has been used in the presentations and discussions of design techniques and the design processes which underlie them.

However, allusions have been made to the concept of classification as an integral part of the design process and classification concepts have been used in some cases, i.e. polyhierarchic and faceted classification. This chapter will describe how classification science applies directly to, and can be used to explain many of the design objectives processes, and techniques.

This chapter will introduce a new set of terms from the classification and information sciences and show how these new terms (at least new as applied to system design) relate to our traditional terminology.

The first and most obvious question which can be asked is how does classification science relate to system design? The answer may also be obvious. Classification science deals with the categorizing and organizing of ideas, concepts, terms, etc., into orderly structures such that any idea, concept or term can be found again, regardless of where, or from what perspective you begin your search. The card catalogs of any public library are perhaps the most commonplace example of such a classification structure. The table of contents to this book might be a more handy example. This book was organized into parts, chapters and sections along the logical flow of system development life cycle, and presents many different concepts. Because some concepts have multiple uses, or were split for presentation purposes into different parts of the book, it might be difficulty to find all that I have said on a particular topic, or to find out whether I have covered the topic at all.

The index to this book was constructed from a list of keywords and phrases extracted from the text of the book, alphabetized, and organized into general terms, medium level detail terms, and very fine detail terms. From this list a reverse index was constructed. Thus the term "data model" was also listed under "model, data".

It is hoped that the combination of the table of contents (organized by topic) and the index (organized by keyword and phrase) will assist the readers in finding what they are looking for, regardless of where or how they start.

Categorization is a Fundamental Process

Categorizing or classifying things is a fundamental process of human existence. The world we live in, business or personal, real or conceptual is composed of myriads of things. Some of these things have very real differences between them, others are somewhat similar and still others are highly similar to each other. The differences or similarities between many of these things are sometimes more artificial than real. Distinctions are made between groups of thingsbecause it is clearer to do so than it is to refer ungrouped things. One reason for making distinctions between things is to put them into groups which are easily manageable or understandable. In almost every case some common characteristic of these things is used to make those distinctions. Sometimes, several characteristics are required in order to make those distinctions. Because things in the real world have many characteristics, any set of characteristics they have in common can be used to make these distinctions, or to group the things. For purposes of illustration only, let us take one of the largest group of things which we deal with, people.

Obviously the world is full of people and it would be impossible to deal with or discuss people in general in any meaningful manner. There are just too many different kinds of people. There are only a few things that you can say about people in general without excluding some of them, i.e. they are alive, they were all born, they will all die, they are all composed of carbon, hydrogen, oxygen and a slew of trace elements, etc.

Once you start adding obvious physical characteristics such as age, sex, race, height, weight, color of hair, color of eyes, etc. you start to place people into groups which are smaller than the whole (people). The more characteristics you use the smaller the number of members in each group, and the more different combinations of characteristics you can use to make up each group. Once you start using characteristics (or values of characteristics) to group things you are categorizing or classifying them.

A definition:

The fewer the characteristics that the members of a group have in common, the more members the group can potentially have.

Classification of Entities

An entity is an unclassified fact of being or a thing. Everything that exists in reality, or in the perception is an entity. This group being to large and too general to do anything meaningful with, it is usual to break that too general class down into five slightly less general classes - people, places, things, concepts and events.

Although the entities have been distinguished into five groups, these groups are also usually too general to work with.

For the purposes of developing a business system design, and in particular the data models, each of these five classes are divided into two, more restrictive, classes, one class containing those people, places, things concepts and events our firm is interested in and one containing those in which it is not interested.

The members of each of the five restricted classes can be further classified into two still further restrictive classes, those the firm must collect data about and those it does not have to collect data about.

We can see from this discussion that the term entity (the highest grouping in the data model) already represents three levels of categorization or grouping before we begin (Figure 16-1).

Returning to the people illustration, we now have a group called people, more specifically people the firm is interested in (for whatever reason) and more specifically people the firm is interested in and about which the firm must collect data or maintain records (for whatever reason).

This is still a fairly large group, because we are interested in different groups of people for different reasons, and each of those different reasons usually dictate that we need to collect specific kinds of data about each group. However since they are all part of a larger group called people they must obviously have certain characteristics in common. This differentiation between data that is common to the larger group and data which is specific to the smaller groups which comprise it (at a minimum the addition of at least one characteristic that can be used to segregate members into the appropriate group) is what gives rise to the hierarchic representation of the classification model.

Hierarchic Representation of Classification

The hierarchic structure of a classification model is somewhat of a misrepresentation, since the classification is in reality a general to specific tree structure, rather than an indication of importance ranking. However what it does adequately represent is the concept of characteristic inheritance. Since each successively smaller group is formed from the members of the larger group which precedes it in the hierarchy, all members must possess the characteristic which formed the parent group as well as those additional characteristics which were used to create its group and those of its siblings. The children of the parent are said to have inherited the characteristics of the parent.

The sum of the characteristics of any given group must include all the characteristics used to form the more general groups in a direct line to the family root or base. The lineage from ancestor to descendent is call the chain (of characteristics).

Just as we group entities into people, places, things, etc., and because entity was too large and too general to handle in a meaningful way, so too we categorize each of those groups into smaller groups, i.e. kinds of people, for ease of handling. These grouping categories may be based upon what the entities are, what they look like, what they do, what purpose they serve, how they are used, etc.,

Once the classification scheme is known, at various times during the design process, the designer can use each of these categorizations or classifications for purposes of discussion, analysis or usage, recognizing that however they decide to group them for a particular purpose, the base population remains the same.

Categorizations may also be used to subdivide only part of a population while excluding the rest.

A given population may be grouped or categorized for various uses by the values of a specific characteristic.

In the real world business model, entities may be concurrently grouped by what they do, by the purpose they serve, how they are used, sometimes by what they look like, and sometimes by what they are.

It is easier to view these smaller groups of the larger population by group name than it is by naming the individuals which comprise the group. Group names are used because although each member of the group is different and uniquely identifiable, the group's members are similarly described, act the same way, or are used for the same purpose. The group names tend to reflect these actions or usages. The group name is in many cases identical to the characteristic used to distinguish the members of the group.

The more general the statement of purpose, the description of the actions or usage, or the characteristic, the more members the group will contain.

Conversely, as these statements of purpose, description of action or usage become more and more restrictive, the narrower the group becomes and the smaller the number of potential members.

Similarly, in the data model, as the definition of the characteristics becomes more and more general the group which can be included under that definition becomes larger. The more specific the characteristic definition or the more extensive the list of characteristics, the smaller the group that can be constructed.

Types, Subtypes and Groups

In many data modeling texts the terms type and subtype are used. We have used these terms throughout this book. A type is a group. There are broadly defined groups and narrowly defined groups. If a type is a broad group, than a subtype is a narrow group within the broad group. However both broad and narrow groups, groups and subgroups, types and subtypes are all still groups

Each group, broad or narrow, large or small, has some number of entities which have a set of characteristics in common. The characteristics may be very general or inclusive, or very specific or exclusive, or some combination of both.

Each entity of any given group may have many characteristics but only share some in common with other members of the group. The number of potential groups which can be formed is determined by the number of identifiable characteristics, the number of characteristics selected, and the number of meaningful combinations of characteristics for each number selected.

Groups may also be formed because they share some characteristics, but not others, i.e. employees with hair and those with none.

As an extension of this idea, groups may also be formed based upon the values of each characteristic shared, i.e. color of hair is red, blonde, brunette. Obviously the possibilities for group formation are endless.

Common Business Entities

Every firm, large or small, deals with multiple different types of entities in the course of conducting its business. Although the names of the various entities will vary from firm to firm, at the most general level they can be grouped into four major categories: People, Places, Physical Things (such as a document, a product, a machine, etc.) and Logical or Legal Things (such as, a corporation, or a business unit). Using these four major categories, we can identify some of the most commonly occurring entities regardless of the enterprise's mission.

People Entities

People Entities fall into three major classes: the people who make up the firm's work force, the people who are its customers or clients, and the people who supply it with raw materials, products, parts, and financial or other services.

Place Entities

Place Entities also fall into three major classes: the places where its services are offered, or its products are made, stored and/or sold, the places where its workers is located, the places where its customers reside, or are otherwise located.

Physical Thing Entities

Physical Thing Entities include: the actual products of the firm, its physical assets (buildings, land, furniture, machinery or other equipment, inventory, supplies, etc.), its financial assets (money, securities, leases, contracts, bank accounts, loans, notes, credit lines, etc.), and the documents, memoranda, accounts, contracts, orders, invoices, statements, checks, vouchers, reports and files which record its business transactions and activities

Logical Thing Entities

Logical or Legal Thing Entities include: the services offered by the firm, the firms who are its customers or clients, the firms or people who supply it with raw materials, products, parts, and financial or other services, the markets within which the firm operates, the governmental and regulatory units under whose jurisdiction the firm operates, and the organizational units into which the firm's workers are grouped for business, operational, and reporting purposes.

Within any given firm it can be expected that most, if not all, of the above entities will be represented. What they are called, how they are defined and described, how they are grouped, and more importantly, what the firm needs to know about them, depends upon the specific business of the firm, its culture, and the business rules, policies, mission statements, charters and procedures which govern what it does and how it operates.

Entity Families

In the real world entity model and in the data entity model, we attempt to use the most general, yet most meaningful, classification or categorization possible. These broad classifications of entities are called families. A real world entity family represents a general class or group consisting of all members who share some minimum set of characteristics in common. A data entity family represents a general class of highly intra-related data about:

  1. all the members of the family,
  2. each characteristic which distinguishes each group of members from each other group of members at each level of categorization
  3. the progression from general to specific, of the characteristic sequence or string for each of the lowest level groups (those with the most characteristics needed to distinguish them from all their siblings and cousins).
  4. The attributes about the members of the group, over and above the characteristics used to form the group (remembering the inheritance of characteristics, and attributes)

  5. a repetition of items 2, 3 and 4 above for each perspective, or way in which the population can be categorized.

Polyhierarchic Classification

If the classification model were only single faceted, and there were only (single) parent to (multiple) child relationships, a simple hierarchy could be used to represent it. In this kind of representation once a leg was created it could split again but never merge with another leg. In other words, if each box in the hierarchy chart were the child of a box above it (its parent), and could be the parent of any number of boxes below it, no child could have more than one parent, and its parent must be within the same family branch. A child can not have more than one parent either from the same or different legs. The simple hierarchy also implies that each level in the hierarchy is of the same generation or degree of generalization.

The real world is more complex than this portrayal. In the real world, children can be the offspring of multiple (sets) of parents, cousins from different branches can produce offspring. In a graphic representation, although there would appear to be a hierarchic relationship, within the hierarchy there would also be a network effect, with legs splitting and rejoining, cousins could be grouped to form pseudo-sibling relationships, etc. This complex network-like hierarchy is called a polyhierarchy and can be used to represent any type of complex family relationship pattern.

In developing the framework entity model, we sought to identify the broad real world entity groups (or families) that populate the internal and external environment of the business. Some of the entities will concern us and some will not. They all have one thing in common, they were derived ultimately from the statements of mission, goals, objectives, etc., which were used to define the strategic and tactical direction of the company and its business processing rules and determinants.

The context of corporate data, is the corporation and the real world entities represented by that data. An order line item, a payment, an employee check, removed from its normal context loses meaning and value. An order line item in context of the complete order and related to the selling unit, the product or service being requested and the customer requesting it, has infinitely more meaning, and value, and can be evaluated and analyzed differently.

Real world entity reference and usage is within the context of the business and its concomitant actions. For representational reasons ease and clarity of definition, and ease of handling and discussion, they are segregated into subsets or groups, but all are nothing more than some aspect of the whole, and therefore unified entity.

Active versus Passive Entities

There are two additional ways in which to classify data entities, as either active or passive. Active data entities are the data entities which change over time, which do things or cause things to be done.

The other category of data entities are of interest, more so because they describe and/or relate entities. These are passive data entities. These data entities are usually fixed in data content, come about full blown, or exist more conceptually than in reality. Some examples of these are job requisitions, job or other related skills, education, locations, organizational units, sales territories, sales offices, and job descriptions, to name a few. These passive entities have static data content, and no meaningful life cycle of their own.

These data entities are describable, but in narrative terms, or as lists of other items, rather than physical things. They are carriers of a concept or an idea. Again, there is overlap between active and passive entities, mostly dependent upon viewpoint. There are no hard and fast rules, but it is important to recognize their existence. It is important in the data modeling portion of systems design to identify both active data (non-static) entities and passive (static) ones. From a business systems design standpoint they behave differently and are used differently.

Process Control Entities

There is one final class or family of data entity which appears in many data models. The family has no name, and thus can be called by any name. We will call it the process control family. Its members are used to remember sequences of data events and to guide processes, and later data events, within the organization.

If we look back to our data framework discussion, and specifically to the the business determinants - missions. charters, goals, objectives, strategies, tactics, etc. we can see that although we can extract the entities from them (the subjects and objects of the statements) and each entity has a name that is meaningful, it is difficult to determine whether the entity represents a group entity or a family of entities. If we scan all of the documentation collected and generated from the analysis phase, we would probably have a myriad of entity groups, and probably only a few entity families and no definitive way to tell which is which. Entity class, entity set and entity family are interchangeable terms within this context.

An entity family consists of all individual and groups of member entities which behave the same way in our organization. All entities which have the same role in the organization or which relate to the environment in the same way usually belong to the same family. Thus the entity family customer - contains all entities who fill the role of customers, that is who order, receive, and or pay for the products or services of the company. This family may also include both past customers, present customers, and potential customers, and these may be both active and passive.

Entity Definition

For each entity (family or otherwise) identified by the design team, a definition must also be created. These definitions will provide valuable insight into where the entity belongs, and what level ofgeneralization it represents. For an entity family, the definition should be broad or general enough to include all members of the family. We must describe the role that the members of this family play. We must define as completely as possible, who and what are the members of this family, or more succinctly, what is the universe of this family. The definition should also permit the determination of who are and are not members of the family. This is usually stated in the form of tests or characteristics. The definitions of groups within a family will always be more restrictive than the family.

Entity Family versus Entity Group Reference

All entity references throughout the system design will either be to a family as a whole, some group of members within some family or to some individual member. All relationships are expressed in terms of an entity relating to another entity. In the business system design models, we assume that because entities relate to the environment, either explicitly or implicitly, they relate to each other as well. These relationships may be strong or weak, active or passive, and in some cases may be of no interest to the company.

Relationships can be viewed in two ways, entity family to entity family (inter-family), and entity to entity or entity group to entity group (intra-family). These relationships, both inter- and intra-family are another manner in which the classification scheme may be represented. To illustrate:

A person may be on the faculty of, be a student of, may be an alumnus of, a trustee of, and a contributor to an educational institution. Each relationship represents a separate, distinct, noteworthy, and more importantly definable characteristic.

Likewise, a person may be a depositor of, a lender to and a borrower from, a mortgagee of, etc., a bank.

Each of these ideas may be represented by either a characteristic used to form an entity group, or by a relationship. These relationships may be direct or thorough some intermediary entity, such as an account entity.

Thus each group is defined in terms of its relationship to some member of another family, rather than through shared characteristics. In this representation however the characteristic is transferred to the relationship. The model's descriptions must explain as fully as possible

  1. each of these intra- and inter-family relationships
  2. the conditions under which each relationship exists, and
  3. the group of entity within each entity family which participates in that relationship.

Since the majority of activity within an organization can be expressed in terms of the relationships between entity and entity, or between entity and company (also an entity, by the way), we can expect that there will be a large number of intra-family relationships in the structure and that relationships between entities both intra- and inter-family will be multiple, conditional, and complex.

Distinguishing between Entities (Entity Roles)

Entity groups, at the family level and below, are primarily developed from the role which the members of each group play in the organization. In some instances however, an entity can play multiple roles. For instance, a company can be a supplier and a customer. While these roles are distinct they are not mutually exclusive. A bank's customer can be a borrower for a car, a depositor, and a mortgagee. Again, non-mutually exclusive. The following general recommendation's for entity class, or entity family identification, recommendations which also govern whether similar entities can, or should be merged into a single family, are as follows:

  1. if the roles are mutually exclusive, i.e. if the entities can play one role in the organization and not any other, define separate entity families.

    The attributes needed about each role will probably be different with little in common between roles. There should be no duplication of individual members in another family.

  2. if the roles are distinct, but not mutually exclusive, merge the entities into a single entity family.

To illustrate:

  1. school rules state that no faculty member may be a student of the school. Although similar in most respects and relationships, these are mutually exclusive entities.
  2. the above school restriction does not apply. The faculty entity and student entity may be merged.

A third alternative is also possible. This states that there are entities who play one role exclusively, but there are some which can play both. This can be handled in the following manner.

Each different role entity is defined into a different family. For each entity that is a member of both families:

  1. define it completely in one family and create a "skeleton" or pseudo-entity in the other. This pseudo-entity would be related to the real entity. The pseudo entity would have all of the characteristics of the second role which are not defined for the first role.
  2. The entity member can be fully defined in both entity families, with a relationship and/or indicator in each which notes that the other exists, and must be kept synchronized.

Entities are assigned to a class, or family, according to the role they play in the company environment. Care must be exercised to restrict the definition of those roles. All entities in the family or class play the same role within the organization. The entities within each family or class are different, in specific, just as people are different in specifics, but alike in their general nature and description.

As with the real world, each entity is unique in that it has its own distinct set of physical attributes (descriptors) operational attributes and relationships. Thus the assumption cannot be made that all data elements within a given attribute of the entity family will be present or active for any given member group of the entity family. Overall however, those elements are needed to describe the entities in the family. Since the description of the family can only be in terms of its family members, the design team must assume that any given entity family member, may have any and thus all possible attributes and relationships.

Just as entities are treated as families, so to the entity attributes are developed for the family. For instance, the demographic data for a doctor and a teacher are different. These in turn are different from the demographics of a school which in turn is different from those of a hospital. However, all of the above have some demographic data which we want to record.

Entity Family Attributes

For each attribute we should ask ourselves, what aspect of a family member does it describe? What data event does does it support?

As the attributes are identified, their definitions and descriptions must answer the following questions:

Is it unique to a family member, or

Is it similar to some other attribute of a more general group within the family with which it can be combined?

Does it describe only the member of the entity family to which it is appended? That is, is it a function of that entity, such that we could place it in an equation of the form:

Could any entity within the family be completely described without this characteristic of data, does the characteristic of data relate only to this entity or could we use it to describe some other entity in another entity family?

One test that is useful is the following: is the characteristic of data such that if it were separate it would need this entity's identifier to be useful or meaningful. If the characteristic of data, needs only this entity's identifier to make it meaningful then it identifies this entity. If the characteristic needs two entity identifiers, this one and another, then it is probably a characteristic of a relationship. An additional question which should be answered is: Would the data in this characteristic, change in meaning out of context of this entity?

In answer to the last question, if the data characteristic describes many entity types equally, or if it would not lose meaning if taken out of the entity context, then the data characteristic may be extracted and is probably a characteristic or attribute of a separate entity. Entities containing these data characteristics are also usually of the kind that might describe other entities. Into this category of entity would fall skills, courses, locations, etc., which are more conceptual in nature and which are used to describe other things. For instance, a skill is used to describe the employee, a job description, a job requisition, etc. Since entities of this type can be described in and of themselves, and are also used to describe other entities, they can and should be isolated out as separate data entities establishing relationships between them and the other entities they describe.

As stated earlier, and restated now for emphasis: The classification of attributes into descriptive, operational or relational is vague at best. The categorization of any given attribute might easily change depending upon usage. Generally speaking data that is more stable and infrequently changed is descriptive, data that is more volatile is operational, and data which is connective describes relationships. The categorization does not affect the usage or structure per se, but assists in the process of identification, segmentation, partitioning and combination. We try to combine entities with like data characteristics. Each characteristic of data should be such that it is the only place we have to reference to obtain data on that aspect of the entity. If all data characteristics are such that they could participate in the entity equation, then they are properly placed. If the characteristic of data can appear in the function equation of more than one entity, then it should be isolated.

Exception Attributes

There is a special category of data characteristics which constitute an exception to this general rule. Generally speaking, they can be termed transient exception data.

To illustrate:

The definitions of each characteristic describe their function, or role - descriptive or operational. The definitions answer the questions: What role does the characteristic play in describing the entity, its actions or our actions against it? What data would we expect to find in this characteristic, if we assume that this, and only this, characteristic described that aspect of the entity. What is the definition of this aspect of the entity? Why is that particular data characteristic needed?

Data Acquisition and Retention

Two other questions must be addressed. They relate to the acquisition and retention of the data. If this characteristic is needed for this entity, do we acquire it in some operational area. On the time line for this entity has the data event, or events, where each characteristic of data originated been identified? Is it a viable data event? What is the level of effort that is necessary to acquire that data? If it is acquired, is it volatile or relatively stable? Is it time dependent, and if so are previous values or occurrences of historical value? These can be determined from our data events, but in our narratives we now address, the more specific problems of how long do we save, prior, or historical data?

The design team must assess, before it assigns a characteristic, whether the assumed data content is available elsewhere. That is, if it were not recorded here, could we recreate it from other data recorded with this or other entities. If it can be re-created, then is it necessary to record it here also, for in effect it is redundant data?

The narratives, for operational characteristics must address the questions, as to what is the minimum level of data necessary to support the function? How does the function relate to the entity?

If data characteristics relate to multiple functions, or multiple data events, what are the identifiers necessary to distinguish a characteristic from its siblings? If this is a repeating characteristic of data, what identifiers do we need, real or internal to distinguish its twins, its multiple parts? How many multiples may there be?

The design team should at this point be capable of creating a structure or schematic which gives pictorial representation to the entity families, groups of entities within each family, and the various facets of classifications of those groups within each family.

They have separated out all of the common data and created separate characteristics which reflect data which can be used commonly to describe both the family in general, and each group within the family. For these families they should have defined relationships or structures such that they can relate both the family members to each other, and members of different families to each other Some of these new characteristics may themselves be described in terms of other descriptive characteristics, which are common to still larger groups. It is a general characteristic of many of these these families of entities that their description is more narrative than elemental. That is, they are conceptual in nature and can only be described by narrative.

Describing Process Control Entities

The final family of entity is one which we termed the process control entity family. Within this family are members that are kinds of events, or time sequenced things.

Within any functioning system, with randomly occurring events, there is a need to "remember" the order in which things happened, or to store the results of actions such that later actions can be taken against them. There is a need to record the results of decisions, or actions which determine which process is to be taken. There is a need to remember the results of tests which once made, govern future actions and which would be tedious to make again. In some cases randomly occurring events must be processed at some later data in a certain order. Our process control entity serves this function of "remembering" time or ordering time. In essence this entity remembers sequences of occurrences by recording lists of trigger identifiers, or entity identifiers.

To illustrate:

To further illustrate:

This conceptual family of members can be used for system and operational status checks, work allocation, process control and procedural control.

Contact Martin Modell   Table of Contents

Data Directed Systems Design - A Professional's Guide
Written by Martin E. Modell
Copyright © 2007 Martin E. Modell
All rights reserved. Printed in the United States of America. Except as permitted under United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a data base or retrieval system, without the prior written permission of the author.