"What makes good content"…
The answer depends on the purpose of your writing and who are you targeting.
For instance, technical brochures are written completely different vs. a blog article. The tonality, the format, choice of words,
You would have to adopt a different writing style.
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(Because you’re writing each for a unique purpose and target audience!)
So, "What makes good content"… There's no real answer.
In the same context, there's no unique answer to even the second part of your question "How do you know if a piece of content is doing well".
However, that said, just to bring you more value, I will assume…
• You're talking about web copies meant for promotional purpose.
• And your purpose is to engage your target audience.
So, now, here are a few tips to make your content good:
1. Write like you’re talking to the reader in person as a friend.
2. Make your copy informal.
3. Don’t try to sell anything through your content just yet. Not at least with your top and middle of the funnel content.
4. Format your content properly. Avoid large blocks of texts. Have small paragraphs. And segment the entire copy in clean headings and sub-headings.
5. Do you ask questions to your readers in your content? You should, if you don’t already.
6. Focus on providing value to the audience. Offer them exactly why they are consuming your content.
7. Mix your text contents with the right amount of images, infographics, and videos.
8. Edit and proofread religiously.
9. Avoid too much information. Respect readers' time.
These are 9 tips to make your content good.
Coming to the next part of your question "How do you know if a piece of content is doing well", it's fairly simple.
You published this piece of content for a purpose or with a goal.
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Ask yourself if the content is fulfilling its purpose. If yes, the content is doing well. If not, it is doing badly.
In my assumption, I assumed that you created the content to drive engagement on the website.
There are a few direct and indirect metrics that can give you the idea of whether it’s driving engagement or not.
• Session duration
• Bounce rate
• Clicks on your call to action
The idea of "good content" and then measuring its ROI -- it’s very subjective.
There's no hard answer for it. You have to figure it out yourself. The key is knowing your purpose and target audience.
Hope this answer helps.