trying to buy tax lien certs- HELP

I am curently looking at buying some Tax Lien Certs now. Those that were not picked up at the sale in Feb in Arizona. My goal is to get the 16% interest return. As I am looking through the lists there are some nice parcels. I have done title searches and find that the comps are strong. (Anywhere from 5K-35K above what is owed) My question is, I am seeing that some of these properties have not had back taxes paid for several years. Is it better to find those properties that have not had the taxes paid on them for one or two years versus a longer period of time? What would be considered a better risk for getting the certificate redeemed so I can make my 16%?

Thanks for responding!

If you do your research and find out properties have not paid in 3 years. Do a driveby and eyeball the property. If you like it, then pay off all the back taxes. I believe you will need to do a couple other things, but you need to pretty much get control. For example reesearch on Maricopa county website, and find out from them what you will need to do to get the house!. Its a fairly simple process. I think you need to send a certiifed letter and get some paralegals involved. I tried doing the same thing but i am in florida, so I was never able to really do my due diligence. Good luck hope this helps you.