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The pair said that the numbers of people contributing to Wikipedia hit a high in early 2007, declined slightly and have since stabilised.
"Every month, some people stop writing, and every month, they are replaced by new people," they wrote.
The confusion arose over the differing definitions of what constitutes an editor. Dr Ortega counted everyone who made one change as an editor giving a total population of three million people.
By contrast, the Wikimedia Foundation counts only people who make five edits or more as an editor. This gives an editing population of about one million people across all languages. Of that total, the English edition of Wikipedia has about 40,000 editors.