Most GA4 users confidently create and present reports like the one below. However, only the top 1% can identify the intricacies and potential issues in this report:

The ‘First User Default Channel Group‘ dimension has a ‘user’ scope. The ‘source/medium’ dimension has the ‘event’ scope.

Combining these two dimensions with different scopes creates a mismatch in the detail level they represent, resulting in inaccurate data.

Pair the “First User Default Channel Group” with the ‘First user source/medium‘ dimension.

But does this really fix the problem? 

Are we now seeing accurate data in our report? 

No.

The other issue with this report is that you can’t query the “First User Default Channel Group” dimension or the “First user source/medium” dimension with the “sessions” metric:

This is because these dimensions and metrics belong to different scopes.

‘First User Default Channel Group’ and ‘First User Source/Medium’ are user-scoped dimensions. 

‘Sessions’ is a session-scoped metric. 

User-scoped dimensions aggregate data across all sessions for a user, while session-scoped metrics focus on individual sessions.

User-scoped dimensions provide a big-picture view of a user, while session-scoped metrics zoom in on individual sessions. 

That’s why mixing user-scoped dimensions with session-scoped metrics can lead to inaccurate data in your reports.

GA4 scopes for dimensions and metrics

GA4 supports four different scopes for dimensions and metrics: user, session, event and item scope. 

Each scope represents a different level of data aggregation and context. 

These scopes should never be mixed in GA4 reports. Mixing them will result in a ‘scope mismatch’ error.

GA4 should have a system to warn users instantly when they mix different scopes.

GA4 can let you query dimensions and metrics with different scopes without showing any error message.

This leads to users creating and presenting inaccurate reports with great confidence. 

My golden rule for accurate GA4 reporting

Remember my golden rule for accurate GA4 reporting:

“In GA4, reports should only be created with all the dimensions and metrics belonging to the same scope.”

Let’s re-create our report so that all the dimensions and metrics belong to the same scope:

Here, the two dimensions (‘Session default channel group’ and ‘Session source/medium’) and the metric (‘Sessions’) have the same ‘Session’ scope.

The scopes in GA4 are hierarchical

Scoping in GA4 refers to the level at which dimensions and metrics are aggregated and analysed.

GA4 supports four different levels (aka scopes): user, session, event and item.

Each scope represents a different level of data aggregation and context, and they should not be mixed in reports to ensure accurate and meaningful data analysis.

The ‘user’ scope is the highest level, followed by the ‘session’, ‘event’ and ‘item’ scopes.

The scopes in GA4 are hierarchical, meaning that data at a higher scope level (e.g., user) encompasses data at lower levels (e.g., session, event, item).

1. User Scope: The highest level encompasses all sessions and events for a user.
2. Session Scope: Encompasses all events within a single session.
3. Event Scope: Focuses on individual user interactions within a session.
4. Item Scope: Specific to individual products or services within e-commerce events.

The importance of scoping in GA4

Understanding GA4 scopes is essential for accurate data analysis and reporting in GA4. 

For example, combining metrics and dimensions from different scopes can lead to inaccurate data and misleading reports.

The concept of scoping has always been present in Google Analytics, but it has become much more crucial and prominent in GA4.

GA4 provides warnings, such as incompatible requests, when you use a combination of dimensions and metrics that should not be queried together:

The GA4 user interface can also prevent you from selecting certain dimensions and metrics by disabling them in exploration reports.

However, as you know by now, GA4 does not always prevent you from querying dimensions and metrics of different scopes. 

GA4 processes data at a more granular level (event-centric) than Universal Analytics’ session-centric approach. 

This increased granularity makes scope mismatches even more impactful on report accuracy.

By focusing on scope matching, you can ensure that your reports reflect user behaviour at the intended level of detail. 

How to quickly determine the scope of any dimension or metric in GA4

This will help you avoid mixing dimensions and metrics of different scopes in your reports.

GA4 data model clearly defines data scopes for dimensions. 

For example:

User-scoped dimensions are usually prefixed with ‘First user’ (like ‘First user source/medium’).

Note: There are some user-scoped dimensions that are not prefixed with ‘First user’ like ‘country’, ‘city’, ‘Signed in with user ID’ etc.

User-scoped dimensions typically represent user attributes that remain relatively constant. 

For example:

• Country.
• City.
• User ID.
• Membership Level.
• Subscription plan.

If a dimension reflects a user characteristic unlikely to change frequently, it most likely has user scope.

Session-scoped dimensions are prefixed with ‘Session’ (like ‘Session source/medium’).

Event-scoped dimensions do not have a prefix (like ‘source/medium’). That’s how you know they are event-scoped.

Item-scoped dimensions in GA4 do not have a specific prefix, but you can choose a descriptive name during creation. 

The built-in item scoped dimensions in GA4 (like ‘Item name’, ‘Item brand’, ‘Item affiliation’, etc) are easy to recognise because the entire name itself signifies the item-related nature of the data.

It is important to remember that the item-scoped dimensions that I listed do have ‘Item’ in their names, but they are not prefixes. It’s just the dimension name.

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GA4 User-scoped dimensions

User-scope means the value of the dimension is calculated and sent once for each user. 

GA4 User-scoped dimensions provide insights into where new users are coming from. They capture data associated with a user’s first interaction with your website or app.

User-scoped dimensions capture and retain information about a user across all their sessions and events.

In other words, the value of a user-scoped dimension usually does not usually change with each session or interaction.

There are two categories of GA4 user-scoped dimensions:

#1 GA4 Built-in user-scoped dimensions – These are ready-to-use dimensions available in the GA4 reporting interface or data API.

#2 GA4 custom user-scoped dimensions – These are user-defined user-scoped dimensions.

To learn more about user-scoped dimensions in GA4, check out this article: GA4 User-scoped dimensions explained.

GA4 User-scoped metrics?

GA4 does not have user-scoped metrics in the traditional sense, even when these metrics aggregate data specific to individual users across all their sessions.

For example, the following metrics seem user-specific but still do not qualify as user-scoped metrics:

Lifetime Value: This metric requires calculating total revenue or other actions across sessions, which can keep increasing as the user interacts more.

Total Purchases, Events, Conversions: These metrics depend on the user’s activity during each session and can grow with new purchases, events triggered, or conversions completed.

Total Sessions: While seemingly user-specific, it reflects the total number of visits, which keeps increasing as the user returns.

For GA4 metrics to be user-scoped, it needs to represent data that remains constant across all user sessions.

The examples I listed (Lifetime Value, Total Purchases, etc.) focus on aggregating data across sessions, but this data itself can change with each session.

GA4 Session-scoped dimensions

Session-scope means the value of the dimension is calculated and sent once for each session. 

This means that each time a user starts a new session, the session-scoped dimensions are recalculated based on the context of that particular session.

GA4 Session-scoped dimensions provide insights into where both new and returning users are coming from when they start new sessions.

Session-scoped dimensions in GA4 primarily capture information specific to the current session, not necessarily across all sessions or user journeys. They provide context about the user’s interaction within that specific visit.

GA4 has only one category of session-scoped dimensions called the GA4 Built-in session-scoped dimensions. 

These are ready-to-use dimensions available in the GA4 reporting interface or data API.

In GA4, you can not create custom session-scoped dimensions.

To learn more about session-scoped dimensions in GA4, check out this article: GA4 Session-scoped dimensions explained. 

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GA4 Session-scoped metrics

GA4 built-in metrics are typically session-scoped. 

This means they focus on quantifiable aspects of user behaviour within individual sessions on your website or app. 

While GA4 Session-scoped metrics don’t provide a single, unchanging value for a user, they can still be very insightful when combined with user-scoped dimensions.

To learn more about session-scoped metrics in GA4, check out this article: GA4 Session-scoped metrics explained.

GA4 Event-scoped dimensions

Event-scope means the value of the dimension is calculated and sent for each event. 

GA4 Event-scoped dimensions provide insights into the context surrounding a specific user event (e.g., purchase, video play). They capture details specific to user actions (events) on your website or app.

Event-scoped dimensions apply only to the event they are associated with, not to the entire session.

There are two categories of GA4 event-scoped dimensions:

#1 GA4 Built-in event-scoped dimensions – These are ready-to-use dimensions available in the GA4 reporting interface or data API.

#2 GA4 custom event-scoped dimensions – These are user-defined event-scoped dimensions.

To learn more about event-scoped dimensions in GA4, check out this article: GA4 Event-scoped dimensions explained.

GA4 Event-scoped metrics

Event scope means the value of a metric is calculated for each event.

Most built-in metrics in GA4 have a session-scope, but for some odd reason GA4 allows creating custom metrics only with event scope.

GA4 has a disconnect between the session-centric nature of its built-in metrics and the event-scoped approach for custom metrics.

While GA4 doesn’t offer built-in session-scoped custom metrics, there are ways to address session-level needs. 

In some cases, you can leverage event data to create custom metrics that indirectly reflect session-like behaviour. 

For example, you can calculate the number of events per session by grouping events based on a unique session identifier.

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GA4 Item-scoped dimensions

The GA4 item-scoped dimensions are used to capture detailed information about individual products or services within e-commerce events. 

There are two categories of GA4 item-scoped dimensions:

#1 GA4 Built-in item-scoped dimensions – These are ready-to-use dimensions available in the GA4 reporting interface or data API.

#2 GA4 custom item-scoped dimensions – These are user-defined item-scoped dimensions.

To learn more about item-scoped dimensions in GA4, check out this article: GA4 Item Scoped Dimensions Explained.

GA4 Item-scoped Metrics

The GA4 item-scoped metrics allow you to analyse the performance of individual products or items within e-commerce events. These metrics are used to measure the characteristics of item-scoped dimensions.

In other words, item-scoped metrics measure how your products perform based on the details captured by the item-scoped dimensions.

Imagine a library with books categorised by genre (item-scoped dimension).  

Metrics like “number of times borrowed” or “average loan duration” (item-scoped metrics) would analyse how individual books perform within each genre category.

Examples of GA4 Item-Scoped Metrics:

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Example to understand the various GA4 Scopes

Imagine you run an online store and want to see how customers find your website.

Consider the following user journey:

1. A customer named Jamie finds your website through a Facebook ad. Jamie clicks on the ad, visits your website, looks at some t-shirts, and signs up for your newsletter. 
2. A few days later, Jamie returns to your website directly, changes the colour of a t-shirt to blue, adds the blue t-shirt to the cart, and makes a purchase. 
3. A week later, Jamie clicks on a Twitter ad, views a white-coloured t-shirt, adds it to the cart, and bookmarks the page. During this third session, Jamie clicks on an internal promotion link to view the t-shirt again. 
4. During the 4th session, Jamie uses the bookmark to return, changes the quantity of the white t-shirt to two, and completes the purchase.

Jamie’s Path:

Session 1: Paid social → first visit → session start → view item → signup

Session 2: Direct → session start → change product color (custom event) → add to cart (key event) → purchase (key event)

Session 3: Paid social → session start → internal promotion with UTM parameters → view item → add to cart (key event)

Session 4: Direct → session start → increase item quantity (custom event) → purchase (key event)

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Results in GA4

User-Scoped Dimensions:

First user source/medium is assigned at the user level and shows the source/medium from the user’s first session.

Session-Scoped Dimensions:

Session source/medium is assigned at the session level and shows what originated the session. Direct entrances are attributed to the last known source/medium.

Event-Scoped Dimensions:

The table below shows the number of key events for each event triggered in the previous examples. 

It also includes the key event paths that the user took to trigger each key event. 

Note: The ‘first_view’ event, ‘view_item’ event, ‘change_product_color’ event, and ‘increase_item_quantity’ events are not marked as key events and thus do not have key event paths.

Item-Scoped Dimensions:

Items are included in the items array within each e-commerce event and capture data at the item level.

To track when Jamie changes the product colour, you might create a custom event like this:

window.dataLayer = window.dataLayer || [];

window.dataLayer.push({

  event: ‘change_product_color’,

  ecommerce: {

    items: [

      {

        item_id: ‘P12345’,

        item_name: ‘T-shirt’,

        item_category: ‘Apparel’,

        item_variant: ‘Size M’,

        product_color: ‘Blue’, // Custom dimension

      }

    ]

  }

});

To track when Jamie increases the item quantity, you might create a custom event like this:

window.dataLayer = window.dataLayer || [];

window.dataLayer.push({

  event: ‘increase_item_quantity’,

  ecommerce: {

    items: [

      {

        item_id: ‘P67890’,

        item_name: ‘White T-shirt’,

        item_category: ‘Apparel’,

        item_variant: ‘Size L’,

        product_color: ‘White’, // Custom dimension

        quantity: 2

      }

    ]

  }

});

The conversion credit for source/medium applies only to events marked as key events. 

In the example below, we used the default data-driven attribution model, which assigns different conversion credit weights to each touchpoint along the user’s path to a key event.