Incorrect 'Views per visit' calculation when using event-based filters

[andyj2022]

Past Issues Searched

• I have searched open and closed issues to make sure that the bug has not yet been reported

Issue is a Bug Report

• This is a bug report and not a feature request, nor asking for self-hosted support

Using official Plausible Cloud hosting or self-hosting?

Self-hosting

Describe the bug

The "Views per visit" metric appears to be calculated incorrectly when an event-level filter (such as a custom property) is applied. While the "Total Pageviews" and "Total Visits" metrics seem to correctly reflect the filtered data, the derived "Views per visit" metric is inflated and misleading.

I suspect the calculation is using the total pageviews from the entire session for any session that matches the filter, instead of using only the pageviews that actually match the filter.

I have attached two screenshots to illustrate this:

imagen.png: The main dashboard without any filters. The calculation is correct: 202,000÷78,800≈2.56.

Filter.png: The dashboard with a filter Property Website is youth applied. The calculation shown is 5.66, but the expected result should be 4,400÷1,700≈2.59.

Screenshots (see below):

1. No Filter (Correct Calculation)

2. With Property Website is youth Filter (Incorrect Calculation)

Detailed Analysis

The core of the issue lies in the data being used for the numerator in the "Views per visit" calculation when a filter is active.

Total Pageviews (4.4k): This number is correct. It counts only the pageview events that match the filter condition (Property Website is youth).

Total Visits (1.7k): This number is also correct. It counts every unique visit (session) in which at least one pageview event matched the filter condition.

Views per visit (5.66): This is where the error occurs. The calculation being performed seems to be dividing the total number of pageviews from those 1.7k visits by the visit count (1.7k), rather than using the filtered pageview count of 4.4k.

The current, incorrect calculation is:
Views per visit=Number of matching visitsTotal pageviews from all matching visits​

The expected, logical calculation should be:
Filtered VisitsFiltered Pageviews​=1,7004,400​≈2.59

Illustrative Example

Consider a single visit consisting of 10 total pageviews.

Only 2 of these pageviews have the property Website is youth.

The other 8 do not.

When the filter is applied, Plausible correctly identifies this visit as matching and counts it:

Total Pageviews: 2 (the ones that match the filter)

Total Visits: 1 (because the session contains at least one matching pageview)

Views per visit: To calculate this, the system appears to take the matching visit and sum up all its associated pageviews (10), not just the 2 filtered ones. This results in a value of 10 for this single visit.

When scaled across thousands of visits, this inflates the numerator and produces the misleading 5.66 figure instead of the expected 2.59.

Root Cause in the Code

The provided source code confirms this behavior. The metric is calculated from the pre-aggregated sessions table (s).

File: defmodule Plausible.Stats.SQL.Expression
Elixir

def session_metric(:views_per_visit, _query) do
  wrap_alias([s], %{
    views_per_visit:
      fragment(
        "greatest(ifNotFinite(round(sum(? * ?) / sum(?), 2), 0), 0)",
        s.sign,
        s.pageviews,  # <-- THIS IS THE PROBLEM
        s.sign
      ),
    __internal_visits: fragment("toUInt32(greatest(sum(sign), 0))")
  })
end

The SQL logic is effectively sum(s.pageviews) / sum(s.sign).

sum(s.sign) correctly counts the total number of visits that match the filter.

However, s.pageviews is a column in the sessions table that stores the total number of pageviews for the entire session, regardless of any event-level filters.

When the query filters for sessions that contain a specific event property, it correctly selects the right sessions. But it then sums the entire s.pageviews value for those selected sessions, leading to the incorrect calculation.

In summary, this is a logical error in how derived metrics are calculated on pre-aggregated session data when event-level filters are applied. The calculation should use the filtered event count, not the total event count from the session.

Expected behavior

The expected, logical calculation should be:
Filtered VisitsFiltered Pageviews​=1,7004,400​≈2.59

Screenshots

Without filter

With filter

Environment

- OS:
- Browser:
- Browser Version: