An election audit is any review conducted after polls close for the purpose of determining whether the votes were counted accurately (a results audit) or whether proper procedures were followed (a process audit), or both.
Both results and process audits can be performed between elections for purposes of quality management, but if results audits are to be used to protect the official election results from undetected fraud and error, they must be completed before election results are declared final.
Election recounts are a specific type of audit, with elements of both results and process audits.
The need for verification of election results
In jurisdictions that tabulate election results exclusively with manual counts from paper ballots, or 'hand counts', officials do not need to rely on a single person to view and count the votes. Instead, valid hand-counting methods incorporate redundancy, so that more than one person views and interprets each vote and more than one person confirms the accuracy of each tabulation. In this way, the manual count incorporates a confirmation step, and a separate audit may not be considered necessary.
However, when votes are read and tabulated electronically, confirmation of the results' accuracy must become a separate process.
Within and outside elections, use of computers for decision support comes with certain IT risks. Election-Day electronic miscounts can be caused by unintentional human error, such as incorrectly setting up the computers to read the unique ballot in each election and undetected malfunction, such as overheating or loss of calibration. Malicious intervention can be accomplished by corrupt insiders at the manufacturer, distributor or election authority, or external hackers who access the software on or before Election Day.
Computer-related risks specific to elections include local officials’ inability to draw upon the level of IT expertise available to managers of commercial decision-support computer systems and the intermittent nature of elections, which requires reliance on a large temporary workforce to manage and operate the computers. Voting machines are usually air-gapped from the internet, but they receive updates from flash drives which do come from the internet, and in any case air-gapped computers are regularly hacked through flash drives and other means. Besides traditional security risks such as lock-picking and phishing attacks, voting machines are often unattended in public buildings the night before the election. This physical access lets outsiders subvert them.
To reduce the risk of flawed Election-Day output, election managers like other computer-dependent managers rely on testing and ongoing IT security. In the field of elections management, these measures take the form of federal certification of the electronic elections system designs, though there is no way to know the certified software is what is actually installed; security measures in the local election officials’ workplaces; and pre-election testing.
A third risk-reduction measure is performed after the computer has produced its output: Routinely checking the computers' output for accuracy, or auditing. Outside elections, auditing practices in the private sector and in other government applications are routine and well developed. In the practice of elections administration, however, the Pew Charitable Trusts stated in 2016, “Although postelection audits are recognized as a best practice to ensure that voting equipment is functioning properly, that proper procedures are being followed, and that the overall election system is reliable, the practice of auditing is still in its relative infancy. Therefore, a consensus has not arisen about what constitutes the necessary elements of an auditing program.”
Routine results audits also support voter confidence by improving election officials' ability to respond effectively to allegations of fraud or error.
Examples of intrusions into election computers
Computers which tally votes or compile election results are known to have been hacked in the US in 2014   and 2016,  Ukraine in 2014, and South Africa in 1994.   Only the Ukraine hack was disclosed immediately, so regular audits are needed to provide timely corrections.
Audit challenges unique to election results
Confirming that votes were credited to the correct candidates’ totals might seem to be a relatively uncomplicated task, but election managers face several audit challenges not present for managers of other decision-support IT applications. Primarily, ballot privacy prevents election officials from associating individual voters with individual ballots. This makes it impossible for election officials to use some standard audit practices such as those banks use to confirm that ATMs credited deposits to the correct account.
Another challenge is the need for a prompt and irrevocable decision. Election results need to be confirmed promptly, before officials are sworn into office. In many commercial uses of information technology, managers can reverse computer errors even when detected long after the event. However, once elected officials are sworn into office, they begin to make decisions such as voting on legislation or signing contracts on behalf of the government. Even if the official were to be removed because a computer error was discovered to have put that official in office, it would not be possible to reverse all the consequences of the error.
The intermittent nature of elections is another challenge. The number of elections managed by an election authority ranges from two per year (plus special elections) in five US states to one every 4 years (plus by-elections) in parliamentary systems like Canada where different election authorities manage national, provincial and municipal elections. This intermittency limits the development of a full-time, practiced workforce, for either the elections or the audits. Turning election audits over to an independent, disinterested professional accounting firm is another option not available to election officials. Because election results affect everyone, including the election officials themselves, truly disinterested auditors do not exist. Therefore, audit transparency is required to provide credibility.
Attributes of a good election results audit
No governing body or professional association has yet adopted a definitive set of best practices for election audits. However, in 2007 a group of election-integrity organizations, including the Verified Voting Foundation, Common Cause, and the Brennan Center for Justice at NYU School of Law collaborated with the American Statistical Association to produce a set of recommended best practices for post-election results audits:
- Transparency: The public must be allowed to observe, verify, and point out procedural mistakes in all phases of the audit. This requires that audit procedures and standards be adopted, in written form, and made available before the election.
- Independence: While the actual work of post-election audits may be best performed by the officials who conduct the elections, the authority and regulation of post-election audits should be independent of officials who conduct the elections.
- Paper Records: Vote-counting in the audit should be performed with hand-to-eye counts of voter-marked, voter-verified paper ballots.
- Ballot Accounting (chain of custody, or internal control): The records used in the audit must be verified to be true and complete records of the election.
- Confirmation of the correct winners: The audits must reach statistical confidence that the computer-tabulated results identified the correct winners.
- Addressing Discrepancies: When discrepancies are found, investigation is conducted to determine cause of the discrepancies.
- Comprehensive: The ballot-sample selection process includes all jurisdictions and all ballot types (e.g., absentee, mail-in and accepted provisional ballots).
- Additional Targeted Samples: The audit includes a limited non-random sample selected on the basis of factors useful for building voter confidence or improving election management, such as Election-Day problems or preliminary results that deviate significantly from historical voting patterns.
- Binding on Official Results: Post-election audits must be completed before election results are declared official and final, and must either verify or correct the outcome.
Current practices in election results auditing in the United States
Considering the recommended best practices above, two states (DC, MA) allow public observation of all steps; three states (MD, NM, VT) have audits done by the Secretary of State, which is partly independent of election day procedures; 18 states hand-count; six states (CO, MD, NC, NM, RI, VA) have good samples for statistical confidence; no states have ways to recover from discrepancies in the chain of custody; other issues are listed in the table below.
Computerization of elections occurred rapidly in the United States following the presidential election of 2000, in which imprecise vote-counting practices played a controversial role, and the subsequent adoption of the Help America Vote Act (HAVA) of 2002. The rapid switch to computerized vote tabulation forced election officials to abandon many pre-automation practices that had been used to verify vote totals, such as the redundancy included in valid hand-counting procedures.
According to information from state profiles provided by Verified Voting, as of mid-2019, only Colorado, New Mexico, the District of Columbia, and (in legislation that will not be fully implemented until 2020) Rhode Island require local election officials to perform audits that:
- are completed promptly, before official results can be certified;
- expand the audit to a state-wide recount of the audited contest(s) whenever the audit detects serious discrepancies in the original sample; and
- are binding on the official results.
If followed by local election officials, such requirements create an opportunity to detect and correct any outcome-altering miscounts (whether caused by accident or fraud) that affected the preliminary Election-Night counts.
An additional 19 states prescribe audits that check a flat percentage, typically between 1 and 3 percent, of voting machines or precincts. These audits can detect problems in the individual voting machines selected for audit, but cannot confirm the correct results except in races with very large margins between the winner and losers.
Finally, 28 states finalize official election results without verifying computer-tabulated vote totals. These states either lack paper ballots, have no audit requirements, or in four states (Florida, Vermont, Virginia, and Wisconsin) allow audits to be delayed until after winners are certified.
Other ways to group the states include that 20 states audit by means of hand counts; others use machines or a mix. Ten states audit all races on the ballot; others generally audit the top race and 1-4 others. In 14 states the audit results are used to revise official winners when there is a discrepancy. Another 10 may revise official results, depending on local judgment, and in 9 states the audit creates a report without changing official results, including large states like California, Florida and Illinois. Only two states use hand counts to audit all races, and use the results to revise winners; both are small, Alaska and West Virginia.
Table of state audit rules
|State||Method of Auditing Ballots||Do Audits Revise Election Outcomes?||Deadline, days after election day||Number of Contests Audited||Level of Discrepancy to Trigger Action||Action if Discrepancy||State Gets Report?||Which Units Are Sampled?||Sample Size||Can Public See Ballot Marks?||What's Done if Some Ballots Are Missing?||Law||Rules||2017 Population, Millions|
|California||Hand count, except no audit of 1/3 of ballots (the ones tallied after election night)||No||Before finalize||All||No rules||Report||Yes||precincts or machines||1%||No||Cal. Elec. Code §336.5, §15360 (West 2015)||40|
|Texas||No audit on paperless machines, which many counties use, hand count otherwise||Maybe||21||6-Sec. of State chooses 3 races + 3 ballot items||Any||Try to determine cause||Yes||precincts||at least 1% or 3, not paperless machines||No||Tex. Election Code Ann. §127.201 (Vernon 2015)||Election Advisory No. 2012-03||28|
|Florida||Count with machines, independent of election, or by hand||No||Before finalize||all by machine, or 1 by hand, randomly chosen by each county||No rules||Report||Yes||precincts||20%-100% by machine or 2% by hand||No||Fla. Stat. Ann. §101.591||21|
|New York||Count with machines different from voting system or by hand||Yes||15||All||Any||Expand sample in stages to full recount. Just the affected office||Yes||machines||3%||No||N.Y. Election Law § 9-211 (McKinney 2015)||9 N.Y. Comp. Rules & Regs. 6210.18||20|
|Pennsylvania||Hand count||Maybe||Before finalize||All||No rules||Expand, starting 2022||No||No rules||lesser of 2% or 2,000 ballots||No||Pa. Cons. Stat. tit. 25 §3031.17 and Stein settlement||Directive Concerning the Use, Implementation and Operation of Electronic Voting Systems||13|
|Illinois||Count with machines the optical ballots, which are common, others by hand or different machine from election||No||Before finalize||All||Any||Report||Yes||precincts + machines||5%||No||Il. Rev. Stat. ch. 10 §5/24A-15, 10 §5/24C-15||13|
|Ohio||Hand count||Yes||Before finalize||3-President or Governor, random state-wide, random county-wide, in even-numbered years||0.5% (0.2% if winning margin under 1%)||Expand sample. Sec. of State may order full recount of county||Yes||machines, precincts or polling places||at least 5% of votes||No||Secretary of State Directive 2014-36, 2015 Election Official Manual||settlement agreement in League of Women Voters, et al. v. Brunner, Case No. 3:05-CV-7309, US N.District of Ohio||12|
|Georgia||Hand count||Yes||Before finalize||1 chosen by SOS, nonrandom||Statistical cutoffs||Investigate + decide||Yes||ballots||risk-limiting.||No||10|
|North Carolina||Hand count||Yes||Before finalize||1-President or state-wide ballot item||"Significant"||Expand sample to whole county, all races||No||precincts||statistician decides||No||No audit||N.C. Gen. Stat. Ann. §163-182.1||10|
|Michigan||Hand count||No||After finalize||1 statewide||No action||Training||Yes||precincts||5%||No||10|
|New Jersey||No audit of DRE. 72% lack paper ballots||0||N.J. Stat. Ann. §19:61-9||9|
|Virginia||Hand count||No||After finalize||?||Any||Analyze||Yes||machines in random fifth of localities||risk-limiting||No||Code of VA 24.2-671.1||8|
|Washington||Hand count or machines different from the election machines||Maybe||Before finalize||any 1 contest on batches or precincts, 2 in RLA, or 3 on in-person machines||Any||investigate + resolve||Yes||batches, machines, or precincts||In most counties: 6 batches. Or county could choose 3 precincts, or 4% of in-person machines (rarely-used in all-mail state, or RLA||No||Wash. Rev. Code Ann. §29A.60.185, §29A.60.170||7|
|Arizona||Hand count, except provisionals and counties where a party refuses||Yes||Before finalize||5-President, random federal, random state-wide, random legislative, ballot measure||Statistical committee chooses cutoffs||Expand sample, up to whole county, for affected office||Yes||precincts||at least 2% or 2 in person and 1% of mail + early||No||Ariz. Rev. Stat. §16-602||7|
|Massachusetts||Hand count||Maybe||14||6-President, state and national Senator and Representative, random ballot question, only in Presidential general||Cast doubt on outcome||Sec. of State may expand audit||Yes||precincts||3%||Yes||Mass. Gen. Law Ann. ch. 54 § 109A||7|
|Tennessee||Count paper ballots with same method as election day, different optical scanner, but 59% lack paper ballots||No||?||1-President or Governor in general||1%||Expand sample to 3% of precincts. Then no action, but audit is evidence in court||No||precincts||1 to 5||No||Tenn. Code Ann. § 2-20-103||7|
|Indiana||If requested by party chair, hand count paper ballots or use machine different from voting system, but 34% lack paper ballots||Yes||12||All||No rules||Correct errors||No||precincts||up to 5% or 5||No||Indiana Code §3-12-5-14, §3-11-13-39||7|
|Missouri||Hand count||Maybe||Before finalize||5-state-wide candidate + ballot issue, legislator, judicial, county||0.50%||Investigate and resolve||Yes||precincts||5%||No||15 Mo. Code of State Regs. §30-10.110||6|
|Maryland||Hand count sample, and independent machines re-count all ballot images from election machines||No||120, after finalize||? by hand, All on images||Any||Investigate and resolve||Yes||precincts and all images||1%-2% by hand, and 100% of images||No||Code of Md. Regs. §33.08.05||6|
|Wisconsin||Hand count, except early, absentee, provisional||No||Before finalize||4-President or Governor, and 3 random state contests, only in general||Any||If no explanation, get manufacturer to investigate||Yes||wards or other districts reporting results||5%||No||Wis. Stat. Ann. §7.08(6)||Wisconsin Elections Commission Voting Equipment Audits||6|
|Colorado||Hand-compare ballots to computer records, and SOS tally of computer records||Maybe||Before finalize||All contests are checked, but sample is enough for only 2 contests: 1 statewide, 1 in each county (in primary: 1/party/county) non-randomly chosen by Sec. of State||Statistical cutoffs||100% hand tally of whichever of the 2 target contests has errors, not others||Yes||ballots||risk-limiting based on target contests, so may be too few for other contests||No||Missing ballots are treated as being cast for all the losers in risk level calculations||Colo. Rev. Stat. §1-7-515||Colo. Sec. of State Election Rule 25||6|
|Minnesota||Hand count||Yes||Before finalize||2-3-Governor + federal||0.5% or 2 votes||Expand samples for Governor + federal up to whole county, and if counties with 10% of state's ballots discover problems in a race, hand-count affected race statewide||Yes||precincts||3% or at least 2-4||No||Minn. Stat. Ann. §206.89||6|
|South Carolina||No audit, though all voters have paper ballots||0||5|
|Alabama||No audit, though the state has paper ballots||0||5|
|Louisiana||No audit, and the state lacks paper ballots||0||5|
|Kentucky||Hand count paper ballots, many counties use paperless machines for voters with disabilities||Yes||Before finalize||?||No rules||No rules||Yes||precincts||3% to 5%||No||Ky. Rev. Stat. §117.383||4|
|Oregon||Hand count, except small precincts||Yes in counties with discrepancies||Before finalize||2-3: heaviest vote in each county, statewide office + ballot measure||0.50%||Expand sample to whole county for that vote tally system, all races||Yes||precincts with 150 or more ballots||3% to 10% depending on margin of victory||No||Or. Rev. Stat. §254.529||4|
|Oklahoma||No audit, though voters without disabilities have paper ballots; voters with disabilities do not||0||4|
|Connecticut||Count with machines or by hand, except early, absentee, provisional||Maybe||Before finalize||3-President or Governor and 2 other races; 20% of races in primaries and municipal ballots||0.5%||Investigate and if needed recount county with mix of hand and machine counts||Yes||voting districts||5%||No||Conn. Gen. Stat. §9-320f||4|
|Iowa||Hand count||No||After canvass||1-President or Governor||No rules||Report||Yes||precincts||?||No||HB 516, enacted in 2017||3|
|Utah||Hand count||No||Before finalize||All||Any||Record reasons||Yes||machines||1%||No||Election Policy Directive from the Office of the Lieutenant Governor||3|
|Arkansas||Hand count||No||60||?||No rules||No rules||public||precincts||statistical||No||3|
|Nevada||Count with machines or by hand||No||7||All||No rules||No rules||Yes||machines||2% or 3%||No||Nev. Admin. Code 293.255, Nev. Rev. Stat. §293.247||3|
|Mississippi||No audit, and most counties lack paper ballots||0||3|
|Kansas||Hand count||Yes||Before finalize||3-4||No rules||resolve or expand||precincts||1%||No||3|
|New Mexico||Hand count||Yes||Before finalize||3-4: federal, governor + closest statewide race||error over 90% of the margin of victory||Expand sample, up to whole state for affected office||Yes||precincts||none in race with over 15% margin of victory, otherwise enough for 90% confidence||No||N.M. Stat. Ann. §1-14-13.2 et seq.||2|
|Nebraska||Hand count if requested by Sec of State, method unclear||Maybe||No rule||3 nonrandom, 1 each federal, state, local||Unclear||Report||Yes||precincts||2%||Unclear||2|
|West Virginia||Hand count||Yes||Before finalize||All||1% or changed outcome||Expand sample to whole county, all races||No||precincts||3%||No||W. Va. Code, §3-4A-28||2016 Best Practices Guide for Canvass and Recount||2|
|Idaho||No audit unless recount happens||0||Idaho Code §34-2313||2|
|Hawaii||Hand count||Yes||Before finalize||3-state-wide, county-wide, district-wide||Any||Expand audit until staff are satisfied with results||Yes||precincts||10%||No||Hawaii Rev. Stat. §16-42, Haw. Admin. Rules § 3-172-102||1|
|New Hampshire||No audit, though all voters have paper ballots||0||1|
|Maine||No audit, though all voters have paper ballots.||0||1|
|Rhode Island||Hand count||Yes||Before finalize||Some, nonrandom||Statistical cutoffs||Expand sample to full recount||Yes||Precinct, batch or ballot||risk-limiting||No||R.I. Stat. Ann. §17-19-37.4||1|
|Montana||Hand count||Yes in precincts audited||Before finalize||4-random federal, state-wide, legislative, ballot measure||0.5% or 5 votes||Expand sample but not full recount except in small counties with up to 7 precincts. Check machines with errors||Yes||precincts||at least 5% or 1||No||Mont. Code Ann. §13-17-503||1|
|Delaware||Hand count||No||After finalize||All on half of sample, 1 statewide contest on half of sample||0.5%||further tests of same machine, sometimes another||Yes||machines and districts||2 machines per county, 3 districts in Wilmington||No||1|
|South Dakota||No audit, though all voters have paper ballots||0||1|
|North Dakota||No audit, though all voters have paper ballots||0||1|
|Alaska||Hand count, except small precincts||Yes||Before finalize||All||1%||Expand sample to whole district (1/40th of state), all races||Yes||large precincts||1 per House district, with at least 5% of district ballots, so about 600 registered voters||No||Alaska Stat. §15.15.430||1|
|District of Columbia||Hand count||Yes||Before finalize||4-random District-wide, 2-ward-wide, 1 any type||0.25% or 20% of margin of victory||Expand sample, up to whole county, for affected office||Yes||precincts + ballots||5%||Yes||D.C. Code Ann. §1-1001.09a||1|
|Vermont||Sec.of State randomly picks 6 towns to re-scan & machine-count all contests Also rule to hand count ballots which were hand counted on election day.||No||30||All||Any||Investigate||Yes||polling places||? in 6 towns||No||17 Vt. Stat. Ann. §2493, §2581 - §2588||1|
|Wyoming||No audit, though all voters have paper ballots||0||1|
Risk-limiting audits are required in Colorado, North Carolina, Rhode Island and Virginia. These store voting machines' interpretation of each ballot ("cast vote record"), collect them centrally over the internet, re-tally them with an independent computer to check totals, and hand-check a sample of the stored paper ballots to check voting machines' interpretations. Samples are big enough to be sure results are accurate, up to an acceptable level of risk, such as 9%. For each audited race, if the original computer interpretations identified the wrong winner, there is a 9% chance in Colorado that the audit will miss it, and the wrong winner will take office. A lower risk limit would let fewer errors through, but would require larger sample sizes. Close races also require larger sample sizes. Colorado audits only a few races, and audits none of the closest races, which need the biggest samples. Auditing all races would require several counties to hand-count thousands of ballots each.
If the samples do not confirm the initial results, more rounds of sampling may be done, but if it appears the initial results are wrong, risk-limiting audits require a 100% hand count to change the result, even if that involves hand-counting hundreds of thousands of ballots.
Colorado notes that they have to be extremely careful to keep ballots in order, or they have to number them, to be sure of comparing the hand counts with the machine records of those exact ballots. Colorado says it has a reliable system to re-tally the records, but it is not yet publicly documented. 
In 2010, the American Statistical Association endorsed risk-limiting audits, to verify election outcomes. With use of statistical sampling to eliminate the need to count all the ballots, this method enables efficient, valid confirmation of the outcome (the winning candidates). In 2011, the federal Election Assistance Commission initiated grants for pilot projects to test and demonstrate the method in actual elections. In 2014, the Presidential Commission on Election Administration recommended the method for use in all jurisdictions following all elections, to reduce the risk of having election outcomes determined by undetected computer error or fraud. In 2017, Colorado became the first state to implement risk-limiting audits statewide as a routine practice during the post-election process of certifying election results.
Ballot scans for 100% audits
Humboldt County Elections Transparency Project, Clear Ballot, and TrueBallot scan all ballots with a commercial scanner, so extensive audits can be done on the scans without damaging paper ballots, without hand-counting, by multiple groups, independently of the election software. Clear Ballot is certified by the US Election Assistance Commission for voting, and they also have an auditing system, ClearAudit. TrueBallot does not currently serve government elections, just private groups. Both use proprietary software, so if it were hacked at the vendor or locally to create false images of the ballots or false counts, then local officials and the public could not check it.
The most open audit system is the Elections Transparency Project, as used in Humboldt County, California, a medium sized county with 58,000 ballots in the 2016 general election. They use a commercial scanner to scan all ballots and put them in a digitally signed file, so true copies of the file can be reliably identified. Ballots with identifying marks are first hand-copied by election staff, to anonymize them, since they are valid under California law. The scanner prints a number on each ballot before scanning it, so the scan can be checked against the same physical ballot later if needed.
The project has written open source software to read the files, and they check all races on all ballots to see if official counts are correct. The first time they scanned and checked, in 2008, they found 200 missing ballots, showing the value of complete checks. Other jurisdictions can similarly scan ballots and use Humboldt's software or their own to audit all races. They need to ensure their in-house counting software is secure and independent of any bugs or hacks in the election software, just as users of risk-limiting audits do.
Reading these scans, with software independent of the election system, is the only practical way to audit a large number of close races, without large hand counts. If scanned on election night, scans are also the only practical way to bypass problems of physical security, since scans are made and digitally signed before ballots are stored, while other audit methods are too slow to do election night. Scanners can process thousands of ballots per hour, and several scanners can operate simultaneously in larger jurisdictions. Once ballots are scanned with a digital signature, they can be analyzed at any convenient time.
Humboldt has gone beyond this internal approach, and decided to release the digitally signed files of ballot images to the public, so others can use their own software, independent of the election system and officials, to check all races on all ballots.   Public release lets losing candidates who mistrust the election officials' security measures do their own checks. Humboldt County believes the scans give their citizens high confidence in the county election results.
The "Brakey method" is a variant of the Humboldt scans, with a public release of electronic ballot images already created by most current US election systems. The Brakey Method requires that the ballot images be connected to their physical ballots through a unique identifier that appears on both digital image and physical ballot. Because ballot images can be hacked if the election software itself is hacked, either at the vendor or elsewhere, ballot images do not provide an independent check unless they can be compared with physical ballots.
Pennsylvania uses these ballot images for its audits, though it does not release them to the public for the public to audit.
Recounts may be considered to be a specific type of audit, but not all audits are recounts. The Verified Voting Foundation explains the difference between audits and recounts: Post-election audits are performed to “routinely check voting system performance…not to challenge to the results, regardless of how close margins of victory appear to be", while "recounts repeat ballot counting (and are performed only) in special circumstances, such as when preliminary results show a close margin of victory. Post-election audits that detect errors can lead to a full recount.” In the US, recount laws vary by state, but typically require recounting 100% of the votes, while audits may use samples. Recounts incorporate elements of both results and process audits.
Hand and machine counts. Current audits in most states involve counting paper ballots by hand, but some states re-use the same machines used in the election. Three states use different machines, to provide some independent check.
Sample sizes. When states audit, they usually pick a random sample of 1% to 10% of precincts to recount by hand or by machine. If a precinct has more than one machine, and they keep ballots and totals separate by machine, they can draw a sample of machines rather than precincts and count all ballots processed by that machine. These samples can identify systematic errors widely present in the election. They have only a small chance of catching a hack or bug which was limited to a few precincts or machines, even though that could change the result in close races.
Number of races. In the random sample, most states audit only a few races, so they can only find problems in those races.
Physical security. Auditing is done several days after the election, so paper ballots and computer files need to be kept securely. North Carolina specifies that no audit is done if ballots are missing or damaged.
Physical security has its own large challenges. No US state has adequate laws on physical security of the ballots. Security recommendations for elections include: starting audits as soon as possible after the election, regulating access to ballots and equipment, having risks identified by people other than those who design or manage the storage, using background checks and tamper-evident seals. However seals on plastic surfaces can typically be removed and reapplied without damage. 
Experienced testers can usually bypass all physical security systems. Security equipment is vulnerable before and after delivery. Insider threats and the difficulty of following all security procedures are usually under-appreciated, and most organizations do not want to learn their vulnerabilities.
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