In case you aren't holding enough variables in your head already, now there are "ChatGPT Core Updates" to monitor.

This is not an official term, but Sistrix evaluated 38 daily samples of 100K ChatGPT responses (total of 3.8M responses & 100M source mentions) and observed two "update events" in May.

Here's their caveat: "We are measuring a correlation. The model identifier switched on May 23 from “GPT-5 mini” to “GPT-5.5”, and simultaneously the citation distribution shifted. Whether the model change alone was the cause, or whether the retrieval or prompt layer was also changed in parallel, cannot be determined from the outside."

Also, they only measured German-language prompts/responses. But official or not, I'm glad they're systematically measuring output and sharing what they see, since ChatGPT isn't announcing anything. Article has more context and analysis of winners/losers.