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Multidimensional analysis
View on WikipediaIn statistics, econometrics and related fields, multidimensional analysis (MDA) is a data analysis process that groups data into two categories: data dimensions and measurements. For example, a data set consisting of the number of wins for a single football team at each of several years is a single-dimensional (in this case, longitudinal) data set. A data set consisting of the number of wins for several football teams in a single year is also a single-dimensional (in this case, cross-sectional) data set. A data set consisting of the number of wins for several football teams over several years is a two-dimensional data set.
Higher dimensions
[edit]In many disciplines, two-dimensional data sets are also called panel data.[1] While, strictly speaking, two- and higher-dimensional data sets are "multi-dimensional", the term "multidimensional" tends to be applied only to data sets with three or more dimensions.[2] For example, some forecast data sets provide forecasts for multiple target periods, conducted by multiple forecasters, and made at multiple horizons. The three dimensions provide more information than can be gleaned from two-dimensional panel data sets.
Software
[edit]Computer software for MDA include Online analytical processing (OLAP) for data in relational databases, pivot tables for data in spreadsheets, and Array DBMSs for general multi-dimensional data (such as raster data) in science, engineering, and business.
See also
[edit]References
[edit]- ^ Maddala, G.S. (2001). Introduction to Econometrics (3rd ed.). Wiley. ISBN 0471497282.
- ^ Davies, A.; Lahiri, K. (1995). "A new framework for testing rationality and measuring aggregate shocks using panel data". Journal of Econometrics. 68 (1): 205–227. doi:10.1016/0304-4076(94)01649-K.
Multidimensional analysis
View on GrokipediaFundamentals
Definition and Overview
Multidimensional analysis (MDA) is a data analysis technique integral to online analytical processing (OLAP) systems, where data is structured into dimensions and measures to enable comprehensive exploration. Dimensions are qualitative attributes that provide contextual categories, such as time periods, geographic locations, or product types, while measures consist of quantitative numerical values, like sales revenue or unit quantities, that are evaluated across these dimensions.[3] This organization reflects natural business perspectives, allowing analysts to consolidate and examine data in ways that reveal patterns and relationships. The origins of multidimensional analysis trace back to the early 1990s, amid the rapid growth of corporate data from gigabytes to terabytes, which outpaced the analytical capabilities of existing database systems. It was coined and formalized by E. F. Codd, the pioneer of the relational database model, in his 1993 technical report "Providing OLAP (Online Analytical Processing) to User-Analysts: An IT Mandate," positioning OLAP—and by extension MDA—as an essential extension of relational databases to support complex, ad-hoc queries for decision-making. Codd emphasized that multidimensional data analysis is a core characteristic of OLAP, designed to empower end-user analysts with intuitive tools beyond mere data storage and retrieval. Unlike traditional one-dimensional analysis, which involves linear queries on flat files or basic relational tables to extract data along a single attribute or sequence, MDA supports simultaneous interrogation from multiple perspectives, uncovering multidimensional interactions that simpler methods overlook.[4] This capability relies on relational databases as a foundational prerequisite, leveraging their structured storage while augmenting it for analytical depth rather than transactional efficiency.Dimensions and Measures
In multidimensional analysis, dimensions represent the categorical attributes that provide contextual perspectives for data examination, such as product, region, or time, allowing users to slice and view data from multiple angles.[5] These attributes enable the organization of data into meaningful viewpoints, reflecting natural analytical paths in business or scientific contexts. Measures, in contrast, are the numerical facts or quantitative values that are analyzed and aggregated across dimensions, such as total sales or average price, serving as the core metrics of interest.[5] Aggregation functions applied to measures include operations like sum (to compute totals), average (for central tendencies), and count (to tally occurrences), which facilitate summarization at various levels of granularity.[5] Dimensions are often structured into hierarchies, consisting of ordered levels that support drill-down (to finer details) and roll-up (to broader summaries) analyses.[5] For instance, a geographic hierarchy might progress from country to state to city, enabling users to navigate from high-level regional overviews to specific urban locales.[6] Similarly, a time dimension could be organized as year > quarter > month, providing temporal context for trend analysis.[5] A representative example is a sales dataset where dimensions include time (with hierarchy year > quarter > month), product (with hierarchy category > subcategory > item), and location (with hierarchy country > state > city), while the measure is revenue, which can be aggregated (e.g., summed) along these dimensions to reveal insights like quarterly sales by product category in specific regions.[5] These elements are typically organized within data cubes to support efficient multidimensional querying.Core Concepts
Multidimensional Data Models
Multidimensional data models provide formal representations of data in relational databases to support analytical queries in multidimensional analysis, organizing information into fact and dimension components for efficient retrieval and exploration. These models, often implemented as schemas, structure data to capture business processes through numeric measures and descriptive attributes, enabling users to analyze data across multiple dimensions such as time, product, and location. Unlike traditional relational models, they emphasize denormalization to optimize query performance over data integrity during updates.[7] The star schema is the simplest and most widely adopted multidimensional data model, featuring a central fact table surrounded by denormalized dimension tables that resemble a star shape. The fact table stores quantitative measures, such as sales amounts or quantities, along with foreign keys that reference the primary keys of the dimension tables; these measures represent the core metrics derived from business events, typically at a granular level like individual transactions. Dimension tables contain descriptive attributes, including hierarchies (e.g., product name, category, and brand), providing context for filtering and grouping the facts. This structure facilitates straightforward joins and supports high-performance queries by minimizing the number of table connections required.[8][9] In contrast, the snowflake schema extends the star schema by normalizing the dimension tables to reduce data redundancy, creating a more complex, multi-level structure where dimension hierarchies are split into separate related tables. For instance, a product dimension might be divided into sub-tables for categories and subcategories, connected through additional foreign keys, which explicitly models relationships within dimensions. While this normalization saves storage space and eases maintenance for slowly changing attributes, it introduces more joins during queries, potentially degrading performance and complicating user navigation compared to the flat star schema. Snowflake schemas are less common in production data marts due to these trade-offs but can be useful in scenarios requiring strict normalization for certain hierarchies.[8][7] The relationship between facts and dimensions in these models is established through foreign keys in the fact table that point to primary keys in the dimension tables, enabling relational joins to combine measures with contextual attributes during analysis. This one-to-many linkage allows facts to be contextualized across multiple dimensions simultaneously, such as aggregating sales by product and region, without embedding all descriptive data directly in the fact table. Joins are optimized in star schemas by using simple integer surrogate keys, ensuring efficient retrieval even with large datasets.[9][7] Conceptually, multidimensional data models differ from normalized OLTP models by prioritizing analytical query speed through denormalization, whereas OLTP designs focus on transaction processing efficiency and data consistency via third normal form (3NF) structures. OLTP models normalize to eliminate redundancy and support frequent updates with minimal anomalies, often resulting in many interconnected tables that slow down complex ad-hoc queries. In multidimensional models, denormalization flattens dimensions to reduce join operations, trading some storage efficiency for faster aggregation and slicing across historical data, which is essential for decision-support systems handling terabyte-scale volumes. This shift supports the core goals of multidimensional analysis by making data more accessible for exploratory queries.[8][7]OLAP Cubes
An OLAP cube, also known as a data cube, is a multi-dimensional array of data that organizes facts or measures at the intersections of multiple dimensions, enabling efficient analytical queries across various perspectives.[10] This structure generalizes traditional aggregation operations like group-by and cross-tabulation, treating each dimension as an axis in an N-dimensional space where cells contain aggregated values.[11] In an OLAP cube, dimensions serve as the axes that define the cube's structure, typically extending beyond three dimensions into hypercubes for complex analyses. For instance, a three-dimensional sales cube might have axes for time (e.g., year, quarter, month), product (e.g., category, item), and region (e.g., country, city), with each cell at their intersection holding a measure such as total sales revenue.[10] This arrangement allows users to view data from different angles without restructuring the underlying dataset, as the cube precomputes aggregates across all combinations of dimension levels.[11] Pre-aggregation is a core property of OLAP cubes, involving the storage of summarized data at multiple granularity levels within the cube to accelerate query performance. Rather than computing aggregates on-the-fly from raw data, the cube materializes subtotals, averages, and other functions for various dimension subsets, such as yearly totals or regional averages, reducing the need for repetitive scans of base facts.[10] This lattice-like organization of aggregates, from finest to coarsest levels, supports rapid navigation and minimizes I/O operations during analysis.[11] High-dimensional OLAP cubes often exhibit sparsity, where many cells contain null or zero values due to the combinatorial explosion of dimension combinations, potentially leading to inefficient storage if fully dense arrays are used. To handle sparsity, techniques focus on representing only non-empty cells, such as sparse array formats that store tuples of dimension indices and measures for non-zero entries, avoiding allocation of empty space.[12] Advanced methods, including wavelet decomposition, further compress sparse cubes by approximating aggregates through multiresolution coefficients, preserving query accuracy while reducing storage by orders of magnitude in datasets with low density (e.g., density < 1%).[12]Operations and Techniques
Basic OLAP Operations
Basic OLAP (Online Analytical Processing) operations enable users to interact with multidimensional data cubes by transforming queries and views to extract insights from complex datasets. These operations—primarily slice, dice, and pivot—allow for dynamic manipulation of data without altering the underlying structure, facilitating efficient analysis in business intelligence and data warehousing environments. Introduced in foundational OLAP frameworks, these techniques reduce the complexity of navigating high-dimensional data by focusing on specific subsets or reorienting perspectives. Slice is a fundamental operation that selects a single value from one dimension, effectively reducing the cube's dimensionality by one to produce a lower-dimensional view. For instance, in a sales cube with dimensions of time (quarters), product (categories), and region (continents), applying a slice for the first quarter (Q1) would fix the time dimension to Q1, resulting in a two-dimensional cross-tabulation of products versus regions showing only Q1 sales figures. This operation is particularly useful for isolating temporal or categorical subsets, enabling focused analysis on a specific timeframe or attribute. Dice extends slicing by selecting multiple ranges or specific values across two or more dimensions, extracting a sub-cube that represents a subset of the original data. Using the same sales cube example, a dice operation might specify Europe as the region, electronics as the product category, and the years 2020-2022 as the time range, yielding a three-dimensional sub-cube with sales measures aggregated for those constraints. This allows analysts to examine interactions between dimensions, such as regional product performance over a multi-year period, without overwhelming detail from irrelevant data. Pivot, also known as rotate, reorients the cube by swapping dimensions between axes, changing the viewpoint of the data visualization without altering the underlying aggregates. In the sales cube, if the initial view displays time on rows and regions on columns with products fixed, pivoting could swap time and regions, showing regions on rows and time on columns for a transposed report. This operation is essential for exploring data from different angles, such as shifting from a product-centric to a geography-centric analysis, and is often performed interactively in OLAP tools to reveal hidden patterns. To illustrate these operations step-by-step on a simplified sales cube:- Initial Cube View: Consider a three-dimensional sales cube with dimensions Time (Q1, Q2, Q3, Q4), Region (North America, Europe, Asia), and Product (Electronics, Apparel), and measure Sales (in millions USD). A full cross-tabulation might appear as:
| Product \ Region | North America | Europe | Asia |
|---|---|---|---|
| Electronics | |||
| Q1 | 10 | 8 | 6 |
| Q2 | 12 | 9 | 7 |
| Q3 | 11 | 10 | 8 |
| Q4 | 13 | 11 | 9 |
| Apparel | |||
| Q1 | 5 | 4 | 3 |
| Q2 | 6 | 5 | 4 |
| Q3 | 7 | 6 | 5 |
| Q4 | 8 | 7 | 6 |
- Slice Example: Slicing on Time = Q1 reduces the cube to a 2D table of Product vs. Region:
| Product \ Region | North America | Europe | Asia |
|---|---|---|---|
| Electronics | 10 | 8 | 6 |
| Apparel | 5 | 4 | 3 |
- Dice Example: Dicing on Region = Europe, Product = Electronics, and Time = Q1 to Q2 yields a 1D or summarized view (e.g., a list or bar chart):
- Q1, Europe, Electronics: 8
- Q2, Europe, Electronics: 9
- Pivot Example: From the Q1 slice table, pivoting swaps Product and Region, resulting in:
| Region \ Product | Electronics | Apparel |
|---|---|---|
| North America | 10 | 5 |
| Europe | 8 | 4 |
| Asia | 6 | 3 |
