Bridging the gap between Google Analytics UI and BigQuery export

Minhaz Kazi, Developer Advocate, Google Analytics – April 2023


"But why won't the numbers match with the UI?"

If you have worked with the BigQuery event export data for your GA4 property, you definitely have asked this question at some point. Or worse - someone else asked you this. And while trying to answer it, you probably have been asked the dreaded followup question:

"And where does it say that?"

With this article, we will try to shed light on both.

Before we go into details of how the numbers vary, it is important to understand the intended purpose of the BigQuery event export data. Google Analytics users send their collected data to GA via one of the collection methods - Google Tag, Google Tag Manager, Measurement Protocol, SDKs, and Data Import. Based on the GA property's settings, Google Analytics does significant value addition to the collected data before it reaches the standard reporting surfaces including standard reports, Explorations, and the Data API. These value additions can include inclusion of Google Signals, modeling, traffic attribution, prediction etc.

The standard reporting surfaces aim to provide the maximum value to GA users at the lowest friction. However, we understand that on the broad spectrum of users, some might want to supplement the value additions by Google Analytics or even do something completely customized. For these users, BigQuery event export is the intended starting point. BigQuery event export will have collected data, which is sent from the client or app to Google Analytics. BigQuery event export will not contain granular data on most value additions mentioned above.

Thus, for a large number of use cases, the standard reporting surfaces and the BigQuery export data aren't expected to be reconcilable when it comes to these value addition parts. If there is internal consistency in both and they match with what you are collecting, you should be good to go.

Now let's get into some of the specific reasons for the differences and explore ways for mitigating them when possible. This article focuses on the BigQuery Daily event export only and not the Streaming export.

Sampling

No, it most likely is not sampling.

For Universal Analytics, sampling in the UI reports was a much discussed topic. However, for GA4, standard reports and the Data API use pre-processed database tables and are unlikely to be sampled. Explorations, on the other hand, load raw event and user-level data directly without relying on pre-processed tables. If your exploration contains more events than the quota, then GA will sample your data to show the Exploration. The quota is 10 million events for standard GA properties and up to 1 billion events for GA360 properties. If you are trying to match your BigQuery export numbers with an Exploration report, ensure that the exploration report is not sampled. Data Sampling in GA4 has further explanation and ways to address sampling in exploration.

Active users

If you count all the users who have logged at least one event on your GA4 property, you will get the Total Users metric. Although the Total Users metric is available in Explorations in GA4 UI, the primary user metric used for reporting in GA4 is Active Users. In GA4 UI and in reports, if only Users is mentioned, that usually refers to Active Users. So when calculating user count from BigQuery data, you will need to filter and keep only the active users to make the numbers comparable to the GA UI. The calculation method will also vary based on your selected Reporting Identity.

Technical implementation

In BigQuery event export data, if you count the number of distinct User IDs, you will get the Total Users count. Here's a sample query that shows both Total Users and New Users based on user_pseudo_id:

-- Example: Get 'Total User' count and 'New User' count.

WITH
  UserInfo AS (
    SELECT
      user_pseudo_id,
      MAX(IF(event_name IN ('first_visit', 'first_open'), 1, 0)) AS is_new_user
    -- Replace table name.
    FROM `bigquery-public-data.ga4_obfuscated_sample_ecommerce.events_*`
    -- Replace date range.
    WHERE _TABLE_SUFFIX BETWEEN '20201101' AND '20201130'
    GROUP BY 1
  )
SELECT
  COUNT(*) AS user_count,
  SUM(is_new_user) AS new_user_count
FROM UserInfo;

To filter out active users, you will need to write a query based on the definition of Active Users. An active user is a user who matches any one of the following criteria:

Query implementations may vary from property to property. In the near future, the BigQuery event export data will contain an additional field (is_active_user) to flag active users directly. Once the field is implemented, you will be able to use the following query to filter for only active users:

-- Example: Get exact and approximate Active User count count.

WITH
  ActiveUsers AS (
    SELECT
    *
    -- Replace table name.
    FROM `bigquery-public-data.ga4_obfuscated_sample_ecommerce.events_*`
    -- Replace date range.
    WHERE _TABLE_SUFFIX BETWEEN '20201101' AND '20201130'
    AND is_active_user = 1
    GROUP BY 1
  )
SELECT
  COUNT(DISTINCT user_pseudo_id) AS exact_active_user_count,
  APPROX_COUNT_DISTINCT(user_pseudo_id) AS approx_active_user_count
FROM ActiveUsers;

HyperLogLog++

Google Analytics uses HyperLogLog++ (HLL++) algorithm to estimate cardinality for common metrics including Active Users and Sessions. That means when you view the unique count of these metrics in the UI or via the API, they are an approximation with a certain precision. In BigQuery, since you have access to the granular data, you can calculate the exact cardinality for these metrics. So metrics can vary by a small percentage. At 95% confidence interval, the precision might be ±1.63% for session count. The precision levels will vary for different metrics and will change according to the confidence intervals. See HLL++ Sketches for the precision levels at different confidence intervals for different precision parameters of HLL++.

Technical implementation

See Unique count approximation in Google Analytics to understand how HLL++ implemented in Google Analytics and how you can replicate the functionality using BigQuery queries.

Data collection delay

The daily export tables are created after GA collects all events for the day. The daily tables can get updated up to 72 hours beyond the date of the table with events that are time-stamped with the date of the table. Read details about this and see examples. This is more of an issue if your Firebase SDK or Measurement Protocol implementation sends in offline or delayed events. Depending on when the standard reporting surface and BigQuery are updated within those 72 hours, you might see differences between them. For such implementation, comparisons should be made on data older than 72 hours.

High-cardinality reports

Let's assume you are viewing a report through standard reports or the Data API. The report surfaces a large amount of data and has dimensions with high cardinality. High cardinality dimensions might cause the report to exceed the cardinality limit for the underlying table. When this happens, Google Analytics will group less frequent values and label them as (other).

Using a simplified and small scale example, if the cardinality limit for the underlying table is 10 row, this is what you can expect to happen:

Simplified example for Ground-truth data vs Aggregate table with other
row

As you can see, the total number of events remains unchanged. However, less frequent values get grouped together and you cannot re-aggregate the table based on any dimension (e.g. you can't take the aggregate table and derive the total event count for a specific city with high precision). The example gets more profound if you filter the aggregate data based on any of the dimensions.

This grouping of the (other) row happens only in the reporting module and the Data API when the report crosses the cardinality limit. If you do your calculations from BigQuery, you will always end up with the ground-truth data - the most granular rows. Read more about the (other) row and best practices on how to avoid it.

Google Signals

Activating Google Signals on your GA4 property has several benefits including deduplicating users across platforms and devices. Assuming user-id and Google Signals is not implemented, if a person views your website on three different web browsers, Google Analytics will count it as three different users and BigQuery export will have three separate user_pseudo_ids. But with Google Signals activated and the person logged into their single Google account in all three browsers, Google Analytics will duplicate one user and show this count in standard reporting surfaces. However, BigQuery will still show three separate user_pseudo_ids. No Google Signals information is available in the BigQuery export. Thus, reports with Google Signals data will most likely have less user count compared to BigQuery export.

The best way to lessen this effect is to implement User-IDs in your GA4 property along with activating Google Signals. This will ensure that the deduplication happens first based on user_id. For signed in users, user_id field will be populated in BigQuery and can be used for calculation purposes. However, for users that are not signed in(i.e., sessions without user_id), Google Signals will still be used for deduplication.

Also note that certain reports in standard reporting surfaces might have thresholding applied and not return certain data. Most information that can be subject to thresholding usually is not available in the BigQuery export.

Consent mode on websites and mobile apps lets you communicate your users’ cookie or app identifier consent status to Google. When visitors deny consent, GA4 fills the data collection gaps with conversion modeling and behavioral modeling. None of the modeled data is available in the BigQuery event export. When consent mode is implemented, BigQuery dataset will contain cookieless pings collected by GA and each session will have a different user_pseudo_id. Due to modeling, there will be differences between the standard reporting surfaces and the granular data in BigQuery. For example, due to behavioral modeling, you might see less number of active users compared to the BigQuery export as modeling might try to predict the multiple sessions from individual consentless users.

Again, to reduce the effect of this, you should implement User-Ids in your GA4 property. user_id and custom dimensions are exported to BigQuery regardless of the consent status of your users.

Traffic attribution data

In BigQuery traffic attribution data is available at user (first visit) and event level. These are the collected data. However, since Google Analytics implements its own attribution model at session level, that information is neither directly available in BigQuery export nor can it be calculated with full accuracy with the available data. Depending on your use case, you can consider joining the BigQuery dataset with any relevant first party data and building your own attribution model. In future, additional collected data for traffic attribution might be available through BigQuery event export.

Common calculation errors

  • Calculation method: When calculating different metrics in BigQuery, ensure you are using the correct methodology. For example:
    • The standard method of counting sessions for Google Analytics 4 properties is counting the unique combinations of user_pseudo_id/user_id and ga_session_id regardless of the timeframe. In Universal Analytics, sessions would reset at midnight. If you follow the UA model, calculate sessions for each day, and add them up to get a total session count, you would be double counting the sessions that span across multiple days.
    • Depending on your selected Reporting Identity, the user count calculation method will have to be updated.
  • Dimension and metric scope: Ensure that your calculations use the correct user, session, item, or event level scope.
  • Time Zone: In BigQuery export, event_date is for the reporting time zone while event_timestamp is an UTC timestamp in microseconds. So ideally, if one uses event_timestamp in a query, it has to be adjusted for the correct reporting time zone when comparing with UI numbers.
  • Data filtering and Export limits: If you have setup Data Filtering for your BigQuery event export or your daily event export volume has exceeded the limit, the BigQuery event export data will not match with the standard reporting surfaces.

WITH all that, there is a bit to UNNEST in this post. Hopefully you can SELECT the right solutions for your DISTINCT project FROM the guidelines here. If you have questions, JOIN the GA Discord server WHERE queries are most welcome!