Attribution models determine how credit for conversions gets distributed across the touchpoints a customer interacts with before converting. If someone finds you through organic search, clicks a Facebook ad three days later, then converts via a direct visit a week after that, the attribution model decides which channel gets the credit. The most common models are last-click (100% credit to the final touchpoint), first-click (all credit to the first interaction), linear (equal credit to every touchpoint), time-decay (more credit to recent touchpoints), and position-based (40% to first and last, 20% split among middle touches). Google Analytics 4 uses data-driven attribution by default when you have enough conversion volume, which uses machine learning to weight touchpoints based on their actual influence. In practice, most businesses see a 15–40% difference in reported channel performance depending on which model they use. Paid search and retargeting look much better under last-click. Organic and content marketing look stronger under first-click or linear. This matters enormously when you're deciding whether to increase your Google Ads budget or invest in SEO. At Ottawa SEO Inc., we typically recommend starting with the default data-driven model in GA4 if you have 400+ conversions per month across your key action. If your volume is lower, position-based gives a reasonable compromise—it credits both discovery and closing touchpoints without ignoring the nurture phase. We also run parallel reports in different models quarterly to spot major discrepancies, especially for clients spending $8K+ monthly across multiple channels. The biggest mistake is assuming one model is "true." Attribution is always a simplification. Cross-device journeys, offline conversions, and dark social (people sharing links in Slack or WhatsApp) break every model. Use attribution to guide decisions, not dictate them. If a channel consistently shows value across multiple models and your customer interviews confirm it, that's signal. If it only looks good in one model, dig deeper before shifting budget.