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| * Researchers have argued that different programming teams can make similar mistakes.<ref name="nvp-exploit">Lajos Nagy, Richard Ford, and William Allen. [https://cs.fit.edu/Projects/tech_reports/cs-2006-04.pdf N-Version Programming for the Detection of Zero-day Exploits]. The 2006 IEEE Topical Conference on Cybersecurity, Daytona Beach, Florida, April 2006.</ref> In 1986, Knight & [[Nancy Leveson|Leveson]] conducted an experiment to evaluate the assumption of independence in NVP, they found that the assumption of independence of failures in ''N''-version programs failed statistically.<ref name="nvp-exploit" /><ref name="knight-leveson">Knight, J. C. and Leveson, N. G. 1986. [http://portal.acm.org/citation.cfm?coll=GUIDE&dl=GUIDE&id=10688 An experimental evaluation of the assumption of independence in multiversion programming]. IEEE Trans. Softw. Eng. 12, 1 (Jan. 1986), 96-109.</ref><ref name="kl-response">Knight, J. C. and Leveson, N. G. 1990. [http://doi.acm.org/10.1145/382294.382710 A reply to the criticisms of the Knight & Leveson experiment]. SIGSOFT Softw. Eng. Notes 15, 1 (Jan. 1990), 24-35.</ref> | * Researchers have argued that different programming teams can make similar mistakes.<ref name="nvp-exploit">Lajos Nagy, Richard Ford, and William Allen. [https://cs.fit.edu/Projects/tech_reports/cs-2006-04.pdf N-Version Programming for the Detection of Zero-day Exploits]. The 2006 IEEE Topical Conference on Cybersecurity, Daytona Beach, Florida, April 2006.</ref> In 1986, Knight & [[Nancy Leveson|Leveson]] conducted an experiment to evaluate the assumption of independence in NVP, they found that the assumption of independence of failures in ''N''-version programs failed statistically.<ref name="nvp-exploit" /><ref name="knight-leveson">Knight, J. C. and Leveson, N. G. 1986. [http://portal.acm.org/citation.cfm?coll=GUIDE&dl=GUIDE&id=10688 An experimental evaluation of the assumption of independence in multiversion programming]. IEEE Trans. Softw. Eng. 12, 1 (Jan. 1986), 96-109.</ref><ref name="kl-response">Knight, J. C. and Leveson, N. G. 1990. [http://doi.acm.org/10.1145/382294.382710 A reply to the criticisms of the Knight & Leveson experiment]. SIGSOFT Softw. Eng. Notes 15, 1 (Jan. 1990), 24-35.</ref> |
| * The weakness of an NVP program lies in the decision algorithm. The question of correctness of an NVP program depends partially on the algorithm the NVX uses to determine what output is "correct" given the multitude of outputs by each individual ''N''-version program.<ref name="liburd-mit" /> In theory, output from multiple independent versions is more likely to be correct than output from a single version.<ref name="liburd-mit" /> However, there is debate whether or not the improvements of ''N''-version development is enough to warrant the time, additional requirements, and costs of using the NVP method.<ref name="liburd-mit" /> In particular, under certain models of relibility and design effort, it has been shown that improvements due to using NPV is less than if all of the effort was concentrated on improving the reliability of a single version.<ref>{{cite journal | vauthors=((Sha, L.)) | journal=IEEE Software | title=Using simplicity to control complexity | volume=18 | issue=4 | pages=20–28 | date= July 2001 | issn=1937-4194 | doi=10.1109/MS.2001.936213}}</ref> | * The weakness of an NVP program lies in the decision algorithm. The question of correctness of an NVP program depends partially on the algorithm the NVX uses to determine what output is "correct" given the multitude of outputs by each individual ''N''-version program.<ref name="liburd-mit" /> In theory, output from multiple independent versions is more likely to be correct than output from a single version.<ref name="liburd-mit" /> However, there is debate whether or not the improvements of ''N''-version development is enough to warrant the time, additional requirements, and costs of using the NVP method.<ref name="liburd-mit" /> In particular, under certain models of relibility and design effort, it has been shown that improvements due to using NPV are less than if all of the effort was concentrated on improving the reliability of a single version.<ref>{{cite journal | vauthors=((Sha, L.)) | journal=IEEE Software | title=Using simplicity to control complexity | volume=18 | issue=4 | pages=20–28 | date= July 2001 | issn=1937-4194 | doi=10.1109/MS.2001.936213}}</ref> |
| * “There has been considerable debate as to realizing the full potential from n-version programming as it makes the assumption that the independence will lead to statistically independent mistakes. Evidence has shown that this premise may be faulty [12].” [http://www.faa.gov/aircraft/air_cert/design_approvals/air_software/media/AR-01-26_COTS.pdf] | * “There has been considerable debate as to realizing the full potential from n-version programming as it makes the assumption that the independence will lead to statistically independent mistakes. Evidence has shown that this premise may be faulty [12].” [http://www.faa.gov/aircraft/air_cert/design_approvals/air_software/media/AR-01-26_COTS.pdf] |
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