MUM (2021): what Google's Multitask Unified Model does in Search

What Google changed#

MUM was not a ranking update. Google announced the Multitask Unified Model on 18 May 2021 as a technology it would bring to Search over time: “We’ll bring MUM-powered features and improvements to our products in the coming months and years.” There was no rollout window and no ranking change tied to the announcement.

In that post, Google’s Pandu Nayak said MUM uses the T5 text-to-text framework and “is 1,000 times more powerful than BERT.” Google wrote that MUM both understands and generates language, is trained across 75 languages and many tasks at once, and is multimodal, handling text and images.

When it rolled out#

The announcement date, 18 May 2021, is the date on this page because it is the date Google first described MUM. Google described MUM’s first application in Search on 29 June 2021. That was a specific task, not a general ranking change: identifying the names people use for COVID-19 vaccines, so Search could show trusted information for all of them.

What is known about its impact#

Google gave one figure, for that first task: “With MUM, we were able to identify over 800 variations of vaccine names in more than 50 languages in a matter of seconds.” Google said that work would otherwise take hundreds of human hours. It published no figures on ranking or traffic, because MUM was not applied to general ranking.

What site owners should do#

Google’s current ranking systems guide lists MUM as an AI system “capable of both understanding and generating language,” and says it is “not currently used for general ranking in Search.” It names specific uses instead, such as vaccine searches and featured snippet callouts. MUM is still listed as an active system, not a retired one.

• There is nothing to optimize for MUM directly.
• Write for people first, as Google’s helpful content guidance describes.
• Cover a topic in the language your audience searches in, clearly and completely.

How to check whether it affected you#

MUM has no ranking rollout to measure against, and Search Console keeps only 16 months of data in any case. If you are reviewing traffic from mid-2021, attribute changes to the dated updates around it instead, such as the June 2021 and July 2021 core updates. The Google Algorithm Updates timeline lists them with their dates.

Related updates#

• BERT (2019) was the language model Google compared MUM against.
• Passage ranking (2021) was another AI system Google applied to Search that year.
• The June 2021 core update rolled out in the same period.