If you're planning to cast your ballot by mail this year rather than voting in person, these are the most common mistakes to avoid so you can ensure your vote is counted.

Why it matters: About 1% of absentee ballots that were cast in the 2016 and 2018 elections were ultimately tossed, according to the U.S. Elections Assistance Commission (EAC). That could translate to hundreds of thousands of uncounted ballots this year — enough to potentially change the outcome of the presidential race.

• President Trump won Wisconsin in 2016 by fewer than 23,000 votes — less than the number that were rejected in the state's presidential primary this year.

Here are the leading reasons ballots are typically rejected,based on recent elections:

1. Missed deadlines

Around a quarter of rejected absentee ballots in 2016 and 2018 were due to ballots arriving too late, according to EAC studies.

What to do: Check your state's postmark and receipt deadlines. If voting by mail, earlier is better than later once you're ready— especially given thereported mail delays in some places.

• If you're not sure it'll make it in time, drop your ballot off in person or at a drop box if available in your area.
• Most states offer online ballot tracking. Click on your state in the interactive graphic above to find how you can track your ballot or contact information for your local elections office.

2. Forgetting to sign

• 20% of rejected ballots in 2016 and 13% in 2018 were missing the voter's signature.

3. Non-matching signature

Some states compare voters' signatures on their absentee ballots to the signatures provided on their registration paperwork as a way to verify their identity, according to Jack Noland, research manager at RepresentUs.

• Some states will also compare to other signatures they have on record. Try to use consistent signatures.
• Myrna Pérez of the Brennan Center for Justice recommends taking your time signing and using a standard color of pen ink.

What to watch: Eighteen states require that voters be notified when there is a missing signature or signature discrepancy, and given an opportunity to correct it, according to the National Conference of State Legislatures.

4. No witness signature

Some states also require a notary or one or two witnesses to sign an absentee ballot in order for it to be counted.

• Alabama, Alaska, Louisiana, Mississippi, Missouri, North Carolina, Oklahoma and Wisconsin have varying witness signature requirements, according to NCSL.
• Virginia, Minnesota and Rhode Islandwaived the requirements due to the coronavirus. A recent court ruling is forcing South Carolina to reimpose its witness requirement, after a district judge struck it down days before.

5. "Other"

It's a catch-all category, but here are some things it may include:

• Voters not being registered or eligible to vote; a ballot missing an important document such as an affidavit or inner envelope; the voter not completing a document or not clearly marking choices; or the voter has already voted.

What to watch: Pennsylvania will now automatically throw out any ballot that doesn't have a second, inner envelope (called a "secrecy envelope"). Advocates hope rules like this get changed, but be aware this could apply in your state.

Between the lines: Voters in Texas, Missouri, Mississippi, Alabama, Louisiana and Indiana face some of the biggest obstacles because of the rules in those states, National Vote At Home Institute CEO Amber McReynolds told Axios.

The bottom line: Rules vary by state — so read your ballot directions carefully and early enough to line up any extra help you need. Vote early. And check with state and local election offices about how you can be sure your vote was counted.

Ursula Perano contributed reporting for this story.