Have you heard about a term called Spreadmart? If you have not, then it is something you should know because your business users might be using spreadmarts and you need to consider them when creating or enhancing your business intelligence solution.
Image was taken from: http://www.startupdonut.co.uk/blog/2012/08/dangers-using-spreadsheets-accounting
What is a spreadmart? Is it another type of data mart or it is a data warehouse? In a way, it can be considered as a data mart because it is something created specifically for one business process or a department by an individual. It is a spreadsheet, example, an Excel workbook maintained by a business user for performing their own analysis with their unique dataset. Business user uses spreadmarts for combining data from multiple sources including existing reports and produces results for her/his analysis.
Although this gives more flexibility to business users, this introduces some issues with collaboration. Therefore it is always better to maintain IT-driven data marts and ask users to use the data mart for creating their data models either as spreadmart or using a modern way like Power BI.