Probably the most confusing metric Unique Events is now deprecated in the Google Analytics world.
If you rarely used it, you may ask what's so confusing? It has a pretty straightforward name and in combination with Total Events metric provides really useful insights. If that is what you think about this metric, then the numbers won't always make sense for you: the Unique Events count will always be less than Total Events for a row (which is expected), however the sum of Unique Events for all rows will not match column total. Furthermore, introducing additional dimensions to the report will increase the discrepancy even further. Have fun making use of the percentage too :)
In fact, the Unique Events metric is calculating sessions where an individual event occurred per dimensions combination rather than unique events! This often leads to the misuse of the metric and to Custom Reports reports which do not really make any sense unless you actuallyrealise what are Unique Events in Google Analytics.
Explanation: the Unique Events metric calculates the number of sessions !) which contained a unique combination of the dimensions used in the report. In the example above, the same session had an event with Event Action Article Loaded and Start Reading, so it was counted in rows number one and number two. The columnTotal however ignored the Event Action dimension and only summarised sessions with corresponding Event Category, resulting in a number less than the grand total.
Using the standard reports from Google Analytics Portal you will generally stay on the safe side; and numbers will make sense. You step into the red zone once you start adding a secondary dimension; and you are in danger when using Custom Reports. Imagine you create a Custom Report and your marketing team is using the Unique Events metric from it. What if, at a later point, you or somebody else(!) changes the report, adding a new dimension to it? Unique Events numbers will immediately mushroom and I guarantee you will have an unpleasant WOW moment!
The same problem exists in Analytics 360 - premium version of Google Analytics as well. However, Analytics 360 customers leverage the full power of BigQuery and don't need to rely on the Google Analytics Portal to aggregate data.
Ok, so isit a good thing that this metric is deprecated? I would say so. By way of replacement, Google is giving us a real Unique Events metric! Even though this metric is not yet officially launched, it is available in the Google Analytics portal and you can use it in your reporting. Please avoid using this Unique Events (new) metric until it is officially announced. It is still in beta and I have seen it providing some strange numbers. Nonetheless, it is a step in the right direction and we can use this metric in our research.
Unique Events (New): counts the number of events with unique Event properties (Event Category, Event Action and Event Label) that occur within a single session
Unique Dimension Combinations (Legacy "Unique Events"): counts the number of unique dimension combination values, which can apply to any dimensions included in a report. What that means is that if you have dimensions A, B, and C in a report, and include this metric, Google Analytics will count the unique number of times the same combination of values occurs for each row of dimension values (A+B+C) in the report.
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Just for reference: Google Analytics Data deprecation policy