Prisa, the company that owns the Spanish newspaper El Pais, is suing Nielsen for revising downwards the number of unique users of ElPais.com.
Nielsen decided that an advertisement for the site, which appeared on other websites, was being wrongly counted as traffic for ElPais.com.
Data from marketing firms like Nielsen are important in determining the amount websites charge for advertising.
A spokesman for Nielsen declined to comment further on the legal action.
Prisa also wants an independent audit of Nielsen's rating methods.
It is claiming compensation for damage caused to it by the downgrade.
Prisa described the downgrade as "unjustifiable" and took on the Washington DC law firm of Howrey LLP to pursue the case.
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