Other websites actively discourage downvoting someone because you disagree. But Stack Exchange is not like other sites.
There are a few reasons why you might disagree with someone's answer. Almost all of them means there's something wrong with the answer.
- You might have misread or misunderstood the answer. Because this can be the case, try to have a second look just so you know you did your due diligence before deciding to downvote.
- The answer might be wrong. Being wrong is the best reason to downvote. As a note, don't flag a question simply because it is incorrect. Moderators are not the arbiters of correctness.
- The answer might be right, but it doesn't have the information that you need to understand why it is right and/or verify that it is right. Even if you know what the right answer says, that's not very useful to you unless you know that it is indeed the right answer. Answers that are not useful are fair game to downvote. This information usually comes in the form of references, but understand that references do not have a 1 to 1 relation with proof. If the answer convinces you for other reasons, feel free to upvote it. If the answer does have references, but they are not convincing, then feel free to downvote. Also, even if you do agree with an answer, if you believe that the answer doesn't contain information that verifies it, then you can still downvote it.
- The answer is an opinion, which is discouraged. The goal of Politcis.SE is to be a resource for people to learn facts. There are many of other places where you can go to find people's opinions. An exception is if the answer is about a notable person's or group's opinion.
Things to keep in mind:
- It is important to understand what your goals are when downvoting and up-voting a post. Answers aren't always simple and can have have multiple aspects, and they can also be incomplete, or a best estimate. If you think one of the above suggestions doesn't apply in a specific case, that is your prerogative.
- All of this post is suggestions. In reality, you are free to decide for yourself whether or not an answer can be upvoted or downvoted based on almost any criteria you want, with the exception of serial voting or revenge voting.
The reason I'm writing this, is because over time, I've noticed a trend where well written answers that aren't necessarily useful, constantly get rewarded. When I come to stack exchange looking for answers, I want to see answers that actually improve my understanding, and it is frustrating to see that being long and/or well-written are accepted as substitutes for being informative, correct, and useful.
In conclusion: Stack Exchange is not like most other sites. Don't be afraid to downvote an answer because you disagree with it. The fact that you still disagree with the answer even after reading it, means that the answer likely failed in it's purpose.