Analysis of 758,191 URLs across 22 probe lists found near-zero URL-level Jaccard similarity between nearly all list pairs (most < 0.01), including between country blacklists; even at hostname level, blacklists share little with each other or with researcher-curated lists like ONI's 12,107-URL list, indicating that any single probe list systematically misses large portions of what is actually censored.

Survival analysis of 423,265 pages with Wayback Machine histories shows pages on politically controversial topics have substantially shorter lifetimes than those on uncontroversial topics; topic change — not just page deletion — must be treated as 'death' for probe-list purposes, since a page that switches topic no longer contains the sensitive material that made it censorship-relevant.

§6.1 evaluation

China's Great Firewall adds sites to its blacklist within hours of their becoming newsworthy and drops them again just as quickly; conversely, Pakistan's pornography crackdown used a rarely-updated blocklist, causing 50% of consumption to shift to unlisted sites. An outdated probe list will therefore underestimate GFW effectiveness and overestimate effectiveness in countries with static lists.

§1 detection

Topic correlation analysis across 2,904 list-topic pairs (585 significant after Bonferroni correction at α = 0.05) shows social media is disproportionately represented in country blacklists relative to the broader web; video-sharing sites are also frequently blocked, likely to suppress political organization, copyright infringement, or competition with local businesses.

§7.1, Table 3 detection

Syria's 2015 blocklist contained a disproportionately large share of software-related sites because censors applied indiscriminate TLD-based blocking of all .il (Israeli) domain names regardless of content, demonstrating that non-topic-based criteria (country-code TLD, ASN) can sweep in entirely unrelated infrastructure and are detectable only through anomaly spot-checks rather than content analysis.

§7.1 detection

The automated probe list generation system discovered 45.79 potentially blocked domains per 1,000 domains crawled, compared to 4.11 for FilteredWeb — over 10× higher efficacy. It uncovered 1,490 potentially blocked domains in crawls of just 71,960 URLs, versus 1,255 blocked domains found by Hounsel et al. in crawls of 1,000,000 URLs, with 1,473 of the 1,490 domains not overlapping with prior work.

2024-tang-automatic §5.5 measurement-platformdns-poisoningip-blocking

VPS-based vantage points in Singapore and India detected censorship patterns similar to 'free' locations, failing to observe blocking known to be enforced by local ISPs following government directives. This occurred because ISP-level censorship is implemented per-carrier rather than centrally, and the VPS provider's ISP did not enforce those blocks — confirmed by re-testing from a residential IP that did observe the expected blocks.

2024-tang-automatic §6.2 measurement-platformip-blocking

The authors' blockpage-based methodology cannot detect transit censorship implemented via TCP RST injection or packet drops, because distinguishing these from transient network errors requires identifying their location on the routing path. As a result, the 8-country, 6-AS finding is explicitly characterized as a lower bound on the true extent of Russian transit censorship.

2023-ortwein-towards §4.1 Gathering Blockpages / §6 Conclusion rst-injectionip-blockingmeasurement-platform

29 of 80 VPN providers — including paid services — configure clients to resolve DNS through third-party public resolvers (Google Public DNS, Cloudflare, OpenDNS, Quad9) rather than provider-operated infrastructure. Three self-hosted solutions (Algo, Streisand, Outline) hardcode public DNS with no easy override, causing connection failures in regions where those services are blocked.

2022-ramesh-vpnalyzer §VI (Fig. 5) ip-blockingmeasurement-platform

ICLab's commercial VPN vantage points reside in data-center ('content') ASes for 41% of monitored networks, which may experience less aggressive censorship than residential ISPs, making VPN-based measurements a systematic lower bound on blocking rates faced by ordinary users. In countries where both VPN and volunteer-operated device (VOD) vantages coexist, identical block pages were observed from both AS types, indicating similar overt blocking policies, but covert IP-based or RST-injection blocking may still differ by AS class.

2020-niaki-iclab §III-B measurement-platformip-blocking

Four OR ports (443, 8443, 444, 9001) account for 82% of all active public bridge fingerprints as of April 2016, down from 95% in March 2013 but still concentrated. Scanning just three of these ports (443, 8443, 9001) is sufficient to deanonymize 71% of all active public bridges. Additionally, CollecTor's published per-bridge usage statistics allow a censor to rank bridges by client count per country and identify the highest-impact OR ports to scan next.

2017-matic-dissecting §V-D, Figure 6, Table IV ip-blockingmeasurement-platform