Answered By: Deborah Kelley-Milburn
Last Updated: Jul 15, 2015     Views: 71

Both the Oxford English and the Merriam-Webster dictionaries list the three versions as interchangeable, and there are examples of their use as both adjectives and adverbs.  Thus, in common practice, they are used interchangeably, although Microsoft Word considers online a spelling error. 

For the grammar purist, the explication at the following web site seems persuasive:

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