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I used to try different methods to increase the Domain Authority but i still didn't get the exact reason why the DA of a page will increase a decrease. So what i do now if keep building quality links and use the social signals as well. I have noticed that the DA of my page has increased gradually. But i do not really care about increasing the DA now.

There are companies that can get rankings that quickly in competitive verticals.  But to do so they'll need to use techniques that are not within Google's guidelines such as using private blog networks or injecting links via hacking other people's sites. If this is a site that you can walk away from should it get penalized, then hiring them might make sense.  I'd want to ask how they are getting links and to make sure that they are not doing anything illegal or anything that involves hacking other websites.  Getting links from a private blog network (if that's what they are doing) is not illegal or immoral, but if Google catches on and the site gets penalized then you may never be able to recover it. If this is a site that you plan to keep for the long run, then it makes more sense to hire an agency that will improve your search presence gradually.  Good SEO usually takes a lot of time. I have been building back-links manually over the last 3-4 months. Be careful. I'm seeing links that Google will likely see as unnatural such as: http://www.linkaddurl.com/Arts___Humanities/Performing_Arts/Business/Real_Estate/?p=6 http://cqycxzfwzx.com/sell-my-house-fast-for-cash-your-request-fulfilled/ You're in a competitive space.  I am betting that those sites above you with spammy links will drop out of site the next time Penguin hits.  If they don't then it's because they're using private blog networks that Google hasn't been able to detect algorithmically and they're prime targets for a manual penalty. My advice for hiring an SEO company would be to ask them exactly what they will do for your site.  Are they going to improve your on page SEO?  If so, how? Are they able to get you links? If so, how? If they tell you that the process is proprietary then it's probably not above board.  If they give you marketing speak like, "We'll leverage your blah blah blah and produce stellar content marketing blah blah blah" without actually giving you a concrete idea of what they'll do then that's not good.  I'd also ask for references.  If they hide behind an NDA that's not good.  Good SEO companies will have people lined up to give them a reference.