 There has been some controversy over whether the Disavow tool has any effect when manipulation has taken place over many years.At the same time, some anecdotal case studies have been presented which suggest that the tool is effective and that former ranking positions can be restored.Negative SEO started to occur following the Penguin update when it became common knowledge that Google would apply penalties for manipulative links; such practices as negative SEO have caused companies to be diligent in monitoring their backlinks to ensure they are not being targeted by hostile competitors through negative SEO services.