Calculating ARV for a purchase

[Bo Kim]

Other than paid websites, what are some ways to calculate ARV other than comparable sales in the past 6 months? Zillow and Trulia's numbers seem way off and lately redfin numbers seem too optimistic as well.

https://www.biggerpockets.com/files/user/jasonscot... (from 2012 by J Scott)

I did a search on the forums but some of these posts seem a bit outdated, so I wasnt sure if there were other methods investors are using in 2018. 

Thanks!