Choosing which response rate to work with is definitely the hardest part and there are so many variables involved.
According to the 2018 DMA Response Rate Report, direct mail response rates come in at 9% to a house list and 5% to a prospect list. The 2017 report showed a response rate of 5.1% to a house list and 2.9% to a prospect list, and the average between 2003 and 2015 was 3.6% to a house list and 1.6% to a prospect list.
House lists comprise people who have already bought or ordered from you.
Prospect lists are your potential customers who are likely to use your services.
The increase over the years is fascinating. I think it's in a large part due to the mailbox being less cluttered and marketers targeting their mailings better due to increased postage costs.