Event Tracking: Measuring What Users Do

The mental model

Event tracking is the practice of instrumenting your application to emit signals when users perform meaningful actions. Instead of just seeing raw page views, you get a detailed log of behaviors like addToCart, startVideo, or submitForm. This transforms anonymous traffic into a story about user intent and engagement.

How it works

You add a JavaScript snippet from an analytics provider, like Google's gtag.js, to your website. Then, within your application's code, you call a function like gtag('event', 'login') when a user performs that action. This function sends the event name and optional parameters (e.g., method: 'Google') to the analytics service. The service then processes and aggregates this data into reports. Events are often categorized: some are collected automatically (session_start), some are standardized for industries (purchase), and some are fully custom.

When to use it

Use event tracking to measure key performance indicators (KPIs) tied to business goals. This is essential for tracking sign-ups, monitoring feature adoption, and measuring revenue from purchases. It's the foundation for building conversion funnels to identify where users drop off in a process, like an onboarding or checkout flow. It's also critical for A/B testing to determine which variant drives more of a desired action.

When not to use it

Avoid tracking every single trivial interaction, as this creates noise, can incur costs, and makes analysis difficult. Never track personally identifiable information (PII) in standard event parameters, as this can violate privacy laws like GDPR. The most common mistake is creating a custom event when a standardized "Recommended Event" already exists. Using the standard purchase event, for example, unlocks pre-built e-commerce reports that a custom user_bought_item event would not.

One canonical example

A user clicks a "Sign Up" button on your site. Your website's JavaScript executes gtag('event', 'sign_up', { 'method': 'email' });. In Google Analytics, this registers a sign_up event. Because sign_up is a Recommended Event, GA can automatically use this data in its pre-built reports on user acquisition. The custom method parameter allows you to further segment sign-ups by whether they used email, Google, or another social provider.