Referral Spam
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Seem to be getting an awful lot of referral spam (Very large percentage), does it cause a problem with SEO or do search engines ignore it?
Is it likely to have a negative effect?
Is it a type of commercial warfare by competitors?
Does anyone have any direct experience or knowledge?
Thanks
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You're certainly not the only one dealing with that issue.
Fortunately it doesn't negatively impact your rankings but it sure does mess with Analytics.
Jared Gardner did a great post on just this subject that covers why it exists and more importantly, how you can block it right here on the Moz site at https://moz.com/blog/how-to-stop-spam-bots-from-ruining-your-analytics-referral-data
And it's probably good to know too that if it's the subject of that type of article ... you're not alone. You know ... because misery loves company.

Hope that helps !
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This is an issue an increasing amount of people are having. To anwer your questions:
- This spam does not hurt your SEO. Most of these are "ghost referrals", meaning they don't actually hit your server, but only show up in analytics. It will not influence how well you will do in search engines, it's merely a pain in the behind when trying to accurately track incoming traffic to your site.
- I'd recommend filtering them out within google analytics via hostname instead.
- Refer to this great moz post regarding this very issue to ensure your issue is taken care of: https://moz.com/blog/stop-ghost-spam-in-google-analytics-with-one-filter
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According to Google, they don't use any Google Analytics related data for rankings - https://www.youtube.com/watch?v=CgBw9tbAQhU
Is it a type of commercial warfare by competitors?
No, it's spam, which is targeted towards your curiosity - "oh, I got this visit from this website. Let me check it out".
The only negative effect is polluting your GA reports and data. Fortunately, there is a way to setup filters and block all ghost and referral spam. Here is an article - https://moz.com/blog/stop-ghost-spam-in-google-analytics-with-one-filter
About experience with it - yes, I have been fighting it for quite long time by now and what I see is that the amount of referral and ghost spam to our website grows like crazy, however rankings are not affected at all. At least, they are growing, accordingly to our SEO work.