I'd like some help regarding dependent and independent clauses in English

[iAteYourDog]

I've been proofreading a friend's thesis, and I've encountered an issue. Punctuation is really friggin' hard, and I can't always figure out whether a sentence needs a comma, so I tried learning more about the issue by looking into the hard rules on when to use a comma and when not to.

According to what I read online, if two independent clauses (groups of words that can be placed out of context while staying correct) are joined together, you are to add a comma in between. For example: "I went to the mall, but forgot to get socks." The thing that I've been struggling with is identifying whether a clause is dependent or independent in practice. For example this following sentence:

"The amount of earners whose earnings are imputed make up less than 1% of the sample for virtually all years after 1951, and it can therefore be concluded that the data for the bottom 99% is of high quality and can be used for mobility measures."

"The amount of earners whose earnings are imputed make up less than 1% of the sample for virtually all years after 1951." This sentence seems like a fairly clear case of an independent clause, as it can easily be seen as its own sentence.

However, the second part of the sentence is in a bit of a gray area for me. "It can therefore be concluded that the data for the bottom 99% is of high quality and can be used for mobility measures."

At first this part seemed like an independent clause as well, but then I spotted the "therefore". "Therefore" refers to another sentence, so it wouldn't really work out of context... Or would it?

So, my question is: Is the second part of the sentence an independent or a dependent clause, and why? And does the sentence require a comma after 1951? I hope someone with some knowledge in this subject can help me with this. Thanks for your time.

[iAteYourDog]

I have also asked this question here: https://forum.language-learners.org/vie ... b58b7decce
And I've gotten some very useful answers. If you share this problem, be sure to head over there and check out the comments.