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This articles provides some good information about when you you look at data presented in graph or chart form
What are Primary and Secondary Sources
Primary sources are original materials. They provide direct evidence or firsthand account of an event, experiment, person, company or object. Examples of primary sources include eyewitness accounts, interviews, autobiographies, drawings, statistical data, and journals articles reporting original research.
Secondary sources describe, summarize, review, interpret, and analyze primary sources. Examples of secondary sources include books and book chapters, newspapers, magazines, trade publications, and journals articles that describe and evaluate original research done by other people.
In business, primary sources provide information about what a company or industry says about itself. Examples include annual reports, financial statements, press releases, interviews, speeches, blog entries or tweets. Secondary sources provide information about what others think about a company or industry. Examples include pre-packaged company, industry or market reports, articles from newspapers, magazines and trade publications. In the context of marketing research, secondary data sources refer to data sources that have been gathered by someone else for another purpose.
How do these support your analysis?
Identify competitors and understand industry dynamics
Provide background information on the company and its products/services
Provide details on forces in the macro-environment (demographic, economic, socio-cultural, technological, political/legal)
Provide data on the market (market size, market share, target markets)
Provide real data (i.e. facts and statistics) to as you develop your marketing strategy and other aspects of your marketing plan.