Sill vs SplitSignal: Two Bayesian Approaches to Different Problems
Read the postDateApr 5, 2026
CategoryGuide
Related reading
SearchPilot is the gold standard for causal SEO testing. Their server-side page splits, neural network forecast model, and published case studies from brands like Skyscanner and Adidas set the bar for rigorous experimentation in traditional search. They have extended their methodology to GEO, measuring how on-site changes affect AI-influenced organic traffic. Sill takes a different approach: measuring AI citations directly across six platforms using statistical controls. The two tools solve different problems, and some teams will benefit from using both. This comparison breaks down the methodology, pricing, and fit for each.
TL;DR
SearchPilot is the gold standard for causal SEO testing: server-side page splitting, neural network forecast model, 95% confidence, enterprise pricing, 30K+ organic sessions/mo minimum. Their GEO testing extension measures AI-influenced organic traffic on variant pages by identifying four levers: ranking for fan-out queries, ranking in AI sub-searches, appearing compellingly in retrieved results, and improving page content for AI extraction. SearchPilot acknowledges that LLM outputs do not constitute search results in the traditional sense; their measurement captures the traffic end of the chain, not the citation or mention itself. Sill measures AI citations directly across six platforms (ChatGPT, Perplexity, Gemini, Google AI Overviews, Claude, Grok) by splitting queries rather than pages: prompts affected by a content change serve as the treatment group, unaffected prompts serve as controls. Per-platform models surface when a change moves one engine but not another. Built-in calibration establishes empirical false positive rates. Minimum requirement: 25 prompts. Pricing: Free (5 prompts) / $90/mo Basic (120 prompts) / $225/mo Pro (360 prompts) / Enterprise (custom). For enterprise ecommerce with high traffic and template pages, SearchPilot provides causal SEO proof. For brands wanting direct AI citation measurement, Sill provides per-platform evidence with statistical controls. Large teams can use both: SearchPilot for page-level organic testing, Sill for entity-level GEO measurement.
SearchPilot splits pages to test SEO changes on organic traffic; Sill splits queries to test GEO changes on AI citations across six platforms.
SearchPilot is an enterprise SEO A/B testing platform. It groups similar pages (product pages, category pages, listing pages) into control and variant buckets, applies changes server-side via proxy or edge integration, and uses a proprietary neural network forecast model to predict what would have happened without the change. Tests typically reach 95% statistical significance within 14 days. The minimum requirements are 30,000+ organic sessions per month and thousands of same-template pages. Pricing is custom and annual, designed for enterprise ecommerce and large publishers.
Sill is a GEO experimentation platform that measures how content changes affect AI citations directly. Instead of splitting pages, Sill splits the queries that mention your brand: prompts that should be affected by a content change serve as the treatment group, while unaffected prompts serve as controls. The system fits per-platform models across ChatGPT, Perplexity, Gemini, Google AI Overviews, Claude, and Grok, and uses calibration to establish empirical false positive rates. The minimum requirement is 25 prompts. Pricing starts at $90/mo for Basic (120 prompts), with a free tier for monitoring (5 prompts) and Pro at $225/mo (360 prompts).
The fundamental difference: SearchPilot measures organic traffic arriving at your pages (including traffic from users who interacted with AI before clicking through). Sill measures whether AI platforms mention your brand more or less frequently after a content change. These are two different signals in the same funnel.
SearchPilot tests on-site changes and measures AI-influenced organic traffic on variant pages, not direct AI citations across platforms.
SearchPilot's GEO testing offering identifies four levers of influence: ranking for new keywords that expand coverage across AI fan-out queries; ranking better in the sub-searches AI systems perform; appearing more compellingly in the search results that AI retrieves; and improving pages so information is more prominently featured in AI outputs. Their insight that AI engines perform a "whole buyer's journey of searches based on a fan-out set of queries" is correct, and testing whether on-site changes improve performance across those queries is a legitimate measurement path.
The scope of what SearchPilot measures is important to understand. Their tests measure whether an on-site change affected the organic traffic arriving at variant pages, including traffic from users who were influenced by AI answers before clicking through. The platform does not query ChatGPT, Perplexity, or Gemini directly to measure whether your brand is mentioned more or less frequently in AI responses. SearchPilot's own blog acknowledges this boundary: "the outputs of LLMs don't constitute 'search results' in the same way that we are used to, and there isn't the same concept of 'ranking' within a conversation."
This matters because AI-influenced organic traffic and direct AI citation are two different signals. A page change might improve your Google ranking for a query that ChatGPT uses in its RAG retrieval, increasing the likelihood that ChatGPT cites you, which in turn drives traffic. SearchPilot measures the last step of that chain (the traffic) but not the intermediate steps (the citation, the mention, the recommendation). If SearchPilot's GEO testing does measure AI citations directly and we have mischaracterized the scope, we invite them to reach out so we can correct this comparison.
SearchPilot splits pages and forecasts traffic with neural networks; Sill splits queries and estimates citation probability with per-platform Bayesian models.
The methodological difference stems from what each tool measures. Google ranks individual pages, so page-level splitting is the natural experimental design for SEO. AI engines synthesize entity-level answers from multiple sources, and their outputs change 63% of the time between consecutive days. You cannot split a brand into control and variant groups, but you can split the queries that mention it. A content change on your pricing page should affect pricing-related prompts but not prompts about your company's history or leadership. The unaffected prompts provide a within-brand control.
| Dimension | SearchPilot | Sill |
|---|---|---|
| Unit of measurement | Page traffic (organic sessions on variant pages) | Entity citation (brand mentions across AI responses) |
| Control mechanism | Page splitting (matched control and variant page groups) | Query splitting (affected vs. unaffected prompts within brand) |
| What is measured | Organic clicks, including AI-influenced visits | Direct AI mentions and citation frequency |
| Statistical model | Neural network forecast with 95% confidence | Bayesian estimation with statistical controls |
| Minimum requirements | 30K+ organic sessions/mo; 1000s of template pages | 25+ prompts |
| Implementation | Server-side proxy or edge integration | No site changes required |
| AI platforms | Indirect (measures AI-influenced organic traffic) | 6 platforms: ChatGPT, Perplexity, Gemini, AI Overviews, Claude, Grok |
| Pricing | Custom annual contracts (enterprise) | Free (5 prompts) / $90/mo / $225/mo / Enterprise |
| Test cycle | Typically 14 days to significance | First experiment: 6-8 weeks; subsequent: 2-4 weeks |
SearchPilot fits enterprise ecommerce needing causal SEO proof; Sill fits brands measuring direct AI citations; large teams may use both for different layers.
Use SearchPilot if you are an enterprise ecommerce brand or large publisher with 30,000+ organic sessions per month and thousands of template pages. SearchPilot answers the question: "Did this on-site change increase organic traffic, including traffic influenced by AI answers?" Their methodology is proven, their client roster is strong, and their server-side implementation avoids the cloaking risks of client-side approaches. For teams that need to justify six-figure content investments with causal evidence tied to traffic, SearchPilot is the gold standard.
Use Sill if you need to measure whether AI platforms are recommending your brand more or less frequently after a content change. Sill answers a different question: "Following these changes, did ChatGPT, Perplexity, and Gemini mention us more often on relevant queries?" The minimum requirement is 25 prompts, with no traffic floor or template page requirements. This makes it accessible to brands that lack the page volume for SearchPilot, and to any team that wants direct measurement of AI visibility rather than an organic traffic proxy.
Use both if you have the budget and scale. SearchPilot can measure the organic traffic impact of on-site changes at the page level. Sill can measure the entity-level AI citation impact of the same (or different) changes across platforms. The two measurements complement each other: SearchPilot tells you whether traffic moved; Sill tells you whether AI citations moved. Together, they provide coverage of both the SEO and GEO layers of the funnel.
For teams earlier in the process, starting with GEO monitoring and recommendations (available on Sill's free tier across all 6 AI platforms) provides baseline visibility data before investing in experimentation. For a full comparison across all testing and monitoring tools, see our guide to GEO testing and experimentation tools in 2026.
Choosing between these tools depends on whether you need to measure organic traffic impact, direct AI citations, or both.
| If you need... | Use this | Why |
|---|---|---|
| Causal SEO testing at enterprise scale | SearchPilot | Server-side page splits, neural network forecast, proven case studies at 95% confidence |
| Direct AI citation measurement with controls | Sill | Per-platform models across 6 AI engines; calibration for false positive rates; $90/mo entry |
| Both SEO traffic proof and AI citation proof | Both | SearchPilot for page-level organic; Sill for entity-level GEO; complementary signals |
Sill monitors your brand across six AI platforms and uses statistical controls to separate content impact from background noise. Free tier includes monitoring, GEO recommendations, and Brand Watchdog.
Request your first analysis today to see where you stand.