Here's the ACTUAL data source:
http://allhomekc.org/wp-content/uploads/2018/05/FINALDRAFT-COUNTUSIN2018REPORT-5.25.18.pdf
It's worth comparing this with the previous City of Seattle survey which had a different methodology:
http://coshumaninterests-wpengine.netdna-ssl.com/wp-content/uploads/2017/04/City-of-Seattle-Report-FINAL-with-4.11.17-additions.pdf
This is also worth considering in the mix. According to the City of Seattle's City Attorney Peter Holmes...
Seattle has seen its homeless population swell, with 4,505 living without shelter in the city and other select areas of King County in 2016 – a 19% increase over 2015. Researchers estimate that over 50% of people with opioid addictions in Seattle are homeless and Seattle’s Navigation Team – composed of outreach workers and police officers specially trained to interface with the homeless population – estimates that 80% of the homeless individuals they encounter in challenging encampments have substance abuse disorders.