Data warehousing is a business analyst's dream—all the information about the organization's activities gathered in one place, open to a single set of analytical tools. But how do you make the dream a reality? First, you have to plan your data warehouse system. You must understand what questions users will ask it (e.g., how many registrations did the company receive in each quarter, or what industries are purchasing custom software development in the Northeast) because the purpose of a data warehouse system is to provide decision-makers the accurate, timely information they need to make the right choices.

Related: Data Warehousing Step-by-Step

To illustrate the process, we'll use a data warehouse we designed for a custom software development, consulting, staffing, and training company. The company's market is rapidly changing, and its leaders need to know what adjustments in their business model and sales practices will help the company continue to grow. To assist the company, we worked with the senior management staff to design a solution. First, we determined the business objectives for the system. Then we collected and analyzed information about the enterprise. We identified the core business processes that the company needed to track, and constructed a conceptual model of the data. Then we located the data sources and planned data transformations. Finally, we set the tracking duration.

Step 1: Determine Business Objectives

The company is in a phase of rapid growth and will need the proper mix of administrative, sales, production, and support personnel. Key decision-makers want to know whether increasing overhead staffing is returning value to the organization. As the company enhances the sales force and employs different sales modes, the leaders need to know whether these modes are effective. External market forces are changing the balance between a national and regional focus, and the leaders need to understand this change's effects on the business.

To answer the decision-makers' questions, we needed to understand what defines success for this business. The owner, the president, and four key managers oversee the company. These managers oversee profit centers and are responsible for making their areas successful. They also share resources, contacts, sales opportunities, and personnel. The managers examine different factors to measure the health and growth of their segments. Gross profit interests everyone in the group, but to make decisions about what generates that profit, the system must correlate more details. For instance, a small contract requires almost the same amount of administrative overhead as a large contract. Thus, many smaller contracts generate revenue at less profit than a few large contracts. Tracking contract size becomes important for identifying the factors that lead to larger contracts.

As we worked with the management team, we learned the quantitative measurements of business activity that decision-makers use to guide the organization. These measurements are the key performance indicators, a numeric measure of the company's activities, such as units sold, gross profit, net profit, hours spent, students taught, and repeat student registrations. We collected the key performance indicators into a table called a fact table.

Step 2: Collect and Analyze Information

The only way to gather this performance information is to ask questions. The leaders have sources of information they use to make decisions. Start with these data sources. Many are simple. You can get reports from the accounting package, the customer relationship management (CRM) application, the time reporting system, etc. You'll need copies of all these reports and you'll need to know where they come from.

Often, analysts, supervisors, administrative assistants, and others create analytical and summary reports. These reports can be simple correlations of existing reports, or they can include information that people overlook with the existing software or information stored in spreadsheets and memos. Such overlooked information can include logs of telephone calls someone keeps by hand, a small desktop database that tracks shipping dates, or a daily report a supervisor emails to a manager. A big challenge for data warehouse designers is finding ways to collect this information. People often write off this type of serendipitous information as unimportant or inaccurate. But remember that nothing develops without a reason. Before you disregard any source of information, you need to understand why it exists.

Another part of this collection and analysis phase is understanding how people gather and process the information. A data warehouse can automate many reporting tasks, but you can't automate what you haven't identified and don't understand. The process requires extensive interaction with the individuals involved. Listen carefully and repeat back what you think you heard. You need to clearly understand the process and its reason for existence. Then you're ready to begin designing the warehouse.

Step 3: Identify Core Business Processes

By this point, you must have a clear idea of what business processes you need to correlate. You've identified the key performance indicators, such as unit sales, units produced, and gross revenue. Now you need to identify the entities that interrelate to create the key performance indicators. For instance, at our example company, creating a training sale involves many people and business factors. The customer might not have a relationship with the company. The client might have to travel to attend classes or might need a trainer for an on-site class. New product releases such as Windows 2000 (Win2K) might be released often, prompting the need for training. The company might run a promotion or might hire a new salesperson.

The data warehouse is a collection of interrelated data structures. Each structure stores key performance indicators for a specific business process and correlates those indicators to the factors that generated them. To design a structure to track a business process, you need to identify the entities that work together to create the key performance indicator. Each key performance indicator is related to the entities that generated it. This relationship forms a dimensional model. If a salesperson sells 60 units, the dimensional structure relates that fact to the salesperson, the customer, the product, the sale date, etc.

Then you need to gather the key performance indicators into fact tables. You gather the entities that generate the facts into dimension tables. To include a set of facts, you must relate them to the dimensions (customers, salespeople, products, promotions, time, etc.) that created them. For the fact table to work, the attributes in a row in the fact table must be different expressions of the same event or condition. You can express training sales by number of seats, gross revenue, and hours of instruction because these are different expressions of the same sale. An instructor taught one class in a certain room on a certain date. If you need to break the fact down into individual students and individual salespeople, however, you'd need to create another table because the detail level of the fact table in this example doesn't support individual students or salespeople. A data warehouse consists of groups of fact tables, with each fact table concentrating on a specific subject. Fact tables can share dimension tables (e.g., the same customer can buy products, generate shipping costs, and return times). This sharing lets you relate the facts of one fact table to another fact table. After the data structures are processed as OLAP cubes, you can combine facts with related dimensions into virtual cubes.

Step 4: Construct a Conceptual Data Model

After identifying the business processes, you can create a conceptual model of the data. You determine the subjects that will be expressed as fact tables and the dimensions that will relate to the facts. Clearly identify the key performance indicators for each business process, and decide the format to store the facts in. Because the facts will ultimately be aggregated together to form OLAP cubes, the data needs to be in a consistent unit of measure. The process might seem simple, but it isn't. For example, if the organization is international and stores monetary sums, you need to choose a currency. Then you need to determine when you'll convert other currencies to the chosen currency and what rate of exchange you'll use. You might even need to track currency-exchange rates as a separate factor.

Now you need to relate the dimensions to the key performance indicators. Each row in the fact table is generated by the interaction of specific entities. To add a fact, you need to populate all the dimensions and correlate their activities. Many data systems, particularly older legacy data systems, have incomplete data. You need to correct this deficiency before you can use the facts in the warehouse. After making the corrections, you can construct the dimension and fact tables. The fact table's primary key is a composite key made from a foreign key of each of the dimension tables.

Data warehouse structures are difficult to populate and maintain, and they take a long time to construct. Careful planning in the beginning can save you hours or days of restructuring.

Step 5: Locate Data Sources and Plan Data Transformations

Now that you know what you need, you have to get it. You need to identify where the critical information is and how to move it into the data warehouse structure. For example, most of our example company's data comes from three sources. The company has a custom in-house application for tracking training sales. A CRM package tracks the sales-force activities, and a custom time-reporting system keeps track of time.

You need to move the data into a consolidated, consistent data structure. A difficult task is correlating information between the in-house CRM and time-reporting databases. The systems don't share information such as employee numbers, customer numbers, or project numbers. In this phase of the design, you need to plan how to reconcile data in the separate databases so that information can be correlated as it is copied into the data warehouse tables.

You'll also need to scrub the data. In online transaction processing (OLTP) systems, data-entry personnel often leave fields blank. The information missing from these fields, however, is often crucial for providing an accurate data analysis. Make sure the source data is complete before you use it. You can sometimes complete the information programmatically at the source. You can extract ZIP codes from city and state data, or get special pricing considerations from another data source. Sometimes, though, completion requires pulling files and entering missing data by hand. The cost of fixing bad data can make the system cost-prohibitive, so you need to determine the most cost-effective means of correcting the data and then forecast those costs as part of the system cost. Make corrections to the data at the source so that reports generated from the data warehouse agree with any corresponding reports generated at the source.

You'll need to transform the data as you move it from one data structure to another. Some transformations are simple mappings to database columns with different names. Some might involve converting the data storage type. Some transformations are unit-of-measure conversions (pounds to kilograms, centimeters to inches), and some are summarizations of data (e.g., how many total seats sold in a class per company, rather than each student's name). And some transformations require complex programs that apply sophisticated algorithms to determine the values. So you need to select the right tools (e.g., Data Transformation Services—DTS—running ActiveX scripts, or third-party tools) to perform these transformations. Base your decision mainly on cost, including the cost of training or hiring people to use the tools, and the cost of maintaining the tools.

You also need to plan when data movement will occur. While the system is accessing the data sources, the performance of those databases will decline precipitously. Schedule the data extraction to minimize its impact on system users (e.g., over a weekend).

Step 6: Set Tracking Duration

Data warehouse structures consume a large amount of storage space, so you need to determine how to archive the data as time goes on. But because data warehouses track performance over time, the data should be available virtually forever. So, how do you reconcile these goals?

The data warehouse is set to retain data at various levels of detail, or granularity. This granularity must be consistent throughout one data structure, but different data structures with different grains can be related through shared dimensions. As data ages, you can summarize and store it with less detail in another structure. You could store the data at the day grain for the first 2 years, then move it to another structure. The second structure might use a week grain to save space. Data might stay there for another 3 to 5 years, then move to a third structure where the grain is monthly. By planning these stages in advance, you can design analysis tools to work with the changing grains based on the age of the data. Then if older historical data is imported, it can be transformed directly into the proper format.

Step 7: Implement the Plan

After you've developed the plan, it provides a viable basis for estimating work and scheduling the project. The scope of data warehouse projects is large, so phased delivery schedules are important for keeping the project on track. We've found that an effective strategy is to plan the entire warehouse, then implement a part as a data mart to demonstrate what the system is capable of doing. As you complete the parts, they fit together like pieces of a jigsaw puzzle. Each new set of data structures adds to the capabilities of the previous structures, bringing value to the system.

Data warehouse systems provide decision-makers consolidated, consistent historical data about their organization's activities. With careful planning, the system can provide vital information on how factors interrelate to help or harm the organization. A solid plan can contain costs and make this powerful tool a reality.