We'd all probably like to believe that the people we argue with will accept our claims and reasons as perfect and complete by themselves, but most readers just won't do that. They want evidence of some sort--facts, examples, statistics, expert testimony, among others--to back up our reasons. Take one more look at the umbrella. An argument without evidence is like an umbrella without a handle. It will keep the rain off your head but it sure is awkward to hold onto.
To be believable and convincing, evidence should satisfy three conditions:
Sufficient: Is there enough evidence to convince a reasonable reader of the reason and the claim?
Credible: Does the evidence match with the readers' experience of the world? If not, does the evidence come from a source that readers would accept as more knowledgeable than they are? Evidence needs to avoid obvious bias.
Accurate: Does the evidence "tell the truth"? Are statistics gathered in verifiable ways from good sources? Are the quotations complete and fair (not out of context)? Are the facts verifiable from other sources?