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Figure 5: Discovering clusters and descriptions in a database
Clustering and segmentation basically partition the database so that each partition or group is similar according to some criteria or metric. Clustering according to similarity is a concept which appears in many disciplines. If a measure of similarity is available there are a number of techniques for forming clusters. Membership of groups can be based on the level of similarity between members and from this the rules of membership can be defined. Another approach is to build set functions that measure some property of partitions ie groups or subsets as functions of some parameter of the partition. This latter approach achieves what is known as optimal partitioning.
Many data mining applications make use of clustering according to similarity for example to segment a client/customer base. Clustering according to optimization of set functions is used in data analysis e.g. when setting insurance tariffs the customers can be segmented according to a number of parameters and the optimal tariff segmentation achieved.
Clustering/segmentation in databases are the processes of separating a data set into components that reflect a consistent pattern of behaviour. Once the patterns have been established they can then be used to "deconstruct" data into more understandable subsets and also they provide sub-groups of a population for further analysis or action which is important when dealing with very large databases. For example a database could be used for profile generation for target marketing where previous response to mailing campaigns can be used to generate a profile of people who responded and this can be used to predict response and filter mailing lists to achieve the best response.
The following is an example of objects that describe the weather at a given time. The objects contain information on the outlook, humidity etc. Some objects are positive examples denote by P and others are negative i.e. N. Classification is in this case the construction of a tree structure, illustrated in the following diagram, which can be used to classify all the objects correctly.
Decision tree structure
Production rules have been widely used to represent knowledge in expert systems and they have the advantage of being easily interpreted by human experts because of their modularity i.e. a single rule can be understood in isolation and doesn't need reference to other rules. The propositional like structure of such rules has been described earlier but can summed up as if-then rules.
Neural networks have broad applicability to real world business problems and have already been successfully applied in many industries. Since neural networks are best at identifying patterns or trends in data, they are well suited for prediction or forecasting needs including:
The structure of a neural network looks something like the following:
Figure 7: Structure of a neural network
The bottom layer represents the input layer, in this case with 5 inputs labels X1 through X5. In the middle is something called the hidden layer, with a variable number of nodes. It is the hidden layer that performs much of the work of the network. The output layer in this case has two nodes, Z1 and Z2 representing output values we are trying to determine from the inputs. For example, predict sales (output) based on past sales, price and season (input).
Each node in the hidden layer is fully connected to the inputs which means that what is learned in a hidden node is based on all the inputs taken together. Statisticians maintain that the network can pick up the interdependencies in the model. The following diagram provides some detail into what goes on inside a hidden node.
Figure 8: Inside a Node
Simply speaking a weighted sum is performed: X1 times W1 plus X2 times W2 on through X5 and W5. This weighted sum is performed for each hidden node and each output node and is how interactions are represented in the network.
The issue of where the network get the weights from is important but suffice to say that the network learns to reduce error in it's prediction of events already known (ie, past history).
The problems of using neural networks have been summed by Arun Swami of Silicon Graphics Computer Systems. Neural networks have been used successfully for classification but suffer somewhat in that the resulting network is viewed as a black box and no explanation of the results is given. This lack of explanation inhibits confidence, acceptance and application of results. He also notes as a problem the fact that neural networks suffered from long learning times which become worse as the volume of data grows.
The Clementine User Guide has the following simple diagram to summarise a neural net trained to identify the risk of cancer from a number of factors.
Example Neural network from Clementine User Guide
the dynamic synthesis, analysis and consolidation of large volumes of multidimensional dataCodd has developed rules or requirements for an OLAP system;
An alternative definition of OLAP has been supplied by Nigel Pendse who unlike Codd does not mix technology prescriptions with application requirements. Pendse defines OLAP as, Fast Analysis of Shared Multidimensional Information which means;
Fast in that users should get a response in seconds and so doesn't lose their chain of thought;
Analysis in that the system can provide analysis functions in an intuitative manner and that the functions should supply business logic and statistical analysis relevant to the users application;
Shared from the point of view of supporting multiple users concurrently;
Multidimensional as a main requirement so that the system supplies a multidimensional conceptual view of the data including support for multiple hierarchies;
Information is the data and the derived information required by the user application.
One question is what is multidimensional data and when does it become OLAP? It is essentially a way to build associations between dissimilar pieces of information using predefined business rules about the information you are using. Kirk Cruikshank of Arbor Software has identified three components to OLAP, in an issue of UNIX News on data warehousing;
A typical customer order entry OLTP transaction might retrieve all of the data relating to a specific customer and then insert a new order for the customer. Information is selected from the customer, customer order, and detail line tables. Each row in each table contains a customer identification number which is used to relate the rows from the different tables. The relationships between the records are simple and only a few records are actually retrieved or updated by a single transaction.
The difference between OLAP and OLTP has been summarised as, OLTP servers handle mission-critical production data accessed through simple queries; while OLAP servers handle management-critical data accessed through an iterative analytical investigation. Both OLAP and OLTP, have specialized requirements and therefore require special optimized servers for the two types of processing.
OLAP database servers use multidimensional structures to store data and relationships between data. Multidimensional structures can be best visualized as cubes of data, and cubes within cubes of data. Each side of the cube is considered a dimension.
Each dimension represents a different category such as product type, region, sales channel, and time. Each cell within the multidimensional structure contains aggregated data relating elements along each of the dimensions. For example, a single cell may contain the total sales for a given product in a region for a specific sales channel in a single month. Multidimensional databases are a compact and easy to understand vehicle for visualizing and manipulating data elements that have many inter relationships.
OLAP database servers support common analytical operations including: consolidation, drill-down, and "slicing and dicing".
OLAP servers have the means for storing multidimensional data in a compressed form. This is accomplished by dynamically selecting physical storage arrangements and compression techniques that maximize space utilization. Dense data (i.e., data exists for a high percentage of dimension cells) are stored separately from sparse data (i.e., a significant percentage of cells are empty). For example, a given sales channel may only sell a few products, so the cells that relate sales channels to products will be mostly empty and therefore sparse. By optimizing space utilization, OLAP servers can minimize physical storage requirements, thus making it possible to analyse exceptionally large amounts of data. It also makes it possible to load more data into computer memory which helps to significantly improve performance by minimizing physical disk I/O.
In conclusion OLAP servers logically organize data in multiple dimensions which allows users to quickly and easily analyse complex data relationships. The database itself is physically organized in such a way that related data can be rapidly retrieved across multiple dimensions. OLAP servers are very efficient when storing and processing multidimensional data. RDBMSs have been developed and optimized to handle OLTP applications. Relational database designs concentrate on reliability and transaction processing speed, instead of decision support need. The different types of server can therefore benefit a broad range of data management applications.