Abstract
We classify data quality problems that are addressed by data cleaning and provide an overview of the main solution approaches. Data cleaning is especially required when integrating heterogeneous data sources and should be addressed together with schema-related data transformations. In data warehouses, data cleaning is a major part of the so-called ETL process. We also discuss current tool support for data cleaning.
1 Introduction
Data cleaning , also called data cleansing or scrubbing, deals with detecting and removing errors and inconsistencies from data in order to improve the quality of data. Data quality problems are present in single data collections, such as files and databases, e.g., due to misspellings during data entry, missing information or other invalid data. When multiple data sources need to be integrated, e.g., in data warehouses, federated database systems or global web-based information systems, the need for data cleaning increases significantly. This is because the sources often contain redundant data in different representations. In order to provide access to accurate and consistent data, consolidation of different data representations and elimination of duplicate information become necessary.
Data warehouses [6, 16] require and provide extensive support for data cleaning. They load and continuously refresh huge amounts of data from a variety of sources so the probability that some of the sources contain ?dirty data? is high. Furthermore, data warehouses are used for decision making, so that the correctness of their data is vital to avoid wrong conclusions. For instance, duplicated or missing information will produce incorrect or misleading statistics (?garbage in, garbage out?). Due to the wide range of possible data inconsistencies and the sheer data volume, data cleaning is considered to be one of the biggest problems in data warehousing. During the so-called ETL process (extraction, transformation, loading), illustrated in Fig. 1, further data transformations deal with schema/data translation and integration, and with filtering and aggregating data to be stored in the warehouse. As indicated in Fig. 1, all data cleaning is typically performed in a separate data staging area before loading the transformed data into the warehouse. A large number of tools of varying functionality is available to support these tasks, but often a significant portion of the cleaning and transformation work has to be done manually or by low-level programs that are difficult to write and maintain.
Federated database systems and web-based information systems face data transformation steps similar to those of data warehouses. In particular, there is typically a wrapper per data source for extraction and a mediator for integration [32, 31]. So far, these systems provide only limited support for data cleaning, focusing instead on data transformations for schema translation and schema integration. Data is not preintegrated as for data warehouses but needs to be extracted from multiple sources, transformed and combined during query runtime. The corresponding communication and processing delays can be significant, making it difficult to achieve acceptable response times. The effort needed for data cleaning during extraction and integration will further increase response times but is mandatory to achieve useful query results.
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2 Data cleaning problems
This section classifies the ma jor data quality problems to be solved by data cleaning and data transformation. As
we will see, these problems are closely related and should thus be treated in a uniform way. Data transformations [26] are needed to support any changes in the structure, representation or content of data. These transformations become necessary in many situations, e.g., to deal with schema evolution, migrating a legacy system to a new information system, or when multiple data sources are to be integrated.
As shown in Fig. 2 we roughly distinguish between single-source and multi-source problems and between schema- and instance-related problems. Schema-level problems of course are also reflected in the instances; they can be addressed at the schema level by an improved schema design (schema evolution), schema translation and schema integration. Instance-level problems, on the other hand, refer to errors and inconsistencies in the actual data contents which are not visible at the schema level. They are the primary focus of data cleaning. Fig. 2 also indicates some typical problems for the various cases. While not shown in Fig. 2, the single-source problems occur (with increased likelihood) in the multi-source case, too, besides specific multi-source problems.
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