Evidence is anything that can be used to prove an argument (a claim) in a debate.
As noted, in debate, evidence really takes the form of a quotation. That evidence in the quote is testimonial evidence from experts.
The amount of “expertise” in debaters evidence will vary, as the quote may come from an expert in the field (a professor of climatology, for example) or a staff writer from a magazine.
In debate, debaters can (and should) debate the quality of particular pieces of evidence. There are a number of ways they can debate that evidence.
Reputation of the source. Some sources have a stronger reputation than other sources. For example, the Washington Post has a stronger reputation than National Enquirer.
Source bias. All sources have a degree of bias, but some are more biased than others. For example, the Global Times is a newspaper published by China’s government and reflects its point of view. The South China Morning Post is published in Hong Kong and has a slightly anti-China bent, though it was recently acquired by China’s Jack Ma, and some say the acquisition was done to reduce the anti-China spin. In the US, the Washington Post is owned by Jack Bezos (Amazon founder) and some say it reflects his points of view on matters.
Qualifications of the author or person a source is quoting. As the person a climate scientist or a staff writer?
Recency of the publication. Is the publication recent or is it older? If it’s older, does it matter that it is older? What has changed that is important since the publication of the piece.
Other Ways to Judge the Strength of the Evidence
Beyond issues related to the source, evidence can be evaluated in a number of ways
How clearly does it say it? Does the evidence come right out and make the argument being made, or does it it hit that it is a possible argument?
Does it say it, or is the debater extrapolating? I once heard a debater defend the idea that a program would work nation-wide and he calculated the statistics to show it would work. In the debate, he presented the evidence as making the nationwide argument, but the evidence only cited one city and he extrapolated it from the one example (decreased X percentage, saved 5 lives) to being extrapolated across the entire US.
Does it say it? Sometimes evidence that teams present doesn’t say anything like they claim it says. If a claim sounds outlandish, this should be questioned by the other team.
Is it anecdotal? An anecdote is one isolated example. Just because something may be good or bad in on instance doesn’t mean it is good or bad in all instances.
Does it exist? Debaters are generally an honest group, especially in the age of the Internet where things can be easily checked, but sometimes debaters do make up evidence.
Is it based on logical fallacies? There are many logical fallacies and those will be covered separately, but a simple example is correlation vs. causation. Simply because two things happened at the same time does not mean one thing caused the other.
Thee more you debate, the more you will think about how to challenge and contest evidence. These are good starts, and you should think carefully about others.
With these evidence tests in mind, you can start comparing evidence —
-My evidence is more recent
-My author is more qualified
-Your evidence doesn’t say that
-I have more evidence
One very important thing to note about evidence is that evidence usually is not directly contradictory but makes different points that need to be resolved. For example, a policy could hurt the economy in one way and help it in another way. The job of the debaters is to argue why it is net harmful or helpful.