It's crucial to research an employer you are applying to or interviewing with. Knowledge of your employer and the wider field will show an interest in the position, initiative to be prepared, and give you more insight and detail to include when answering questions.
The first source of information about a company will probably be its website. Be sure to look for the "about us" section, as well as any news, media, blog, or press pages. You want to form an idea of how the company presents itself to its customers, and how it communicates its value and its work.
Be sure to find the company or organization on social media as well. This may provide more up to date communications from the organization, and give you more insight into current projects or initiatives.
Mergent Intellect, subscribed to by Cuyahoga County Public Library, offers industry reports with sample questions. Though not exclusively intended for employment interviews, they may spark an idea of something to ask in an interview or to research about the company or industry.
Mergent Intellect
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Industry reports (which contain additional helpful information and insights) can be accessed either by selecting "First Research" or by searching for the company or organization of interest, and then selecting "Industry Details" and selecting a report.