When it comes to list building quality over quantity is mostly based on the traffic. Meaning you should focus on getting a quality lead/prospect instead of focusing on sending a lot of general traffic that is not niche related.
But, you need a good balance. For instance, if you just send 5 quality visitors to a landing page you might not even get 1 optin, obviously depending on a lot of factors such as squeeze page copy, design, offer etc.
So quantity does play a role as well, the more quality leads you send to your optin page the higher chance you have to get the results you want. But that doesn't make quantity important over quality, all I'm saying is that you have to send a good quantity number of quality traffic to your landing pages in order to reach your goal. If that even makes sense
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For instance, you have to spend more time or money getting 100 quality leads who're interested in your offer instead of targeting 10000 people who're not interested or have little interest.
If it was quality then what's the definition of a quality list?
A quality list is a list of email subscribers who don't need much convincing to convert into buying customers, this could be people who have bought a similar product, people searching for a similar product or service via a search engine or a name that keeps popping up in social networks, forums, offline events and online portals. You know someone is a quality lead because they follow everything that has to do with the particular niche you're into.
To put it in short, a quality email list consist of subscribers who open your emails, read them, click links and buy the offers you promote. Even when they need help with anything they would rather email you first before going anywhere for help. That's a quality email list.
and how can I improve the quality of a list?
The first thing you have to make sure is that you're getting quality traffic of people who're really into what you're promoting. Then you can tweak and split test to improve conversions later on because you need traffic to do all the necessary improvements. If you're buying traffic then make sure it's not cheap or non niche related traffic always invest in quality traffic even if it's $1 per click just as long as you get a good ROI though.
Never underestimate cheap traffic though, there are people who're doing a good job out there and saying cheap traffic doesn't work is not fair. Some of these traffic portals/services can surprise you.
Hope that answers your questions.