You compare AI visibility across two dates the way you used to compare rankings. Last month’s screenshot against this week’s, Reddit looks like it fell off, and the gap gets treated as the engine moving. We wanted to know whether ChatGPT had actually dropped Reddit, or whether the two dates were sitting on different questions.
So we ran the comparison on our own benchmark. We track a set of brands across the assistants and ask the same buying questions repeatedly, a sample rather than a survey of the web. The early window is the 14 days from 28 July to 10 August 2026. The late one is 23 August to 5 September 2026, also 14 days.
A cell is one platform plus one brand plus one question, and we only counted a cell if it was present in both windows. If that trio is not on both sides, we left it out, which leaves the same questions and the same brands on both ends. Google AI Mode was measured, but only 17 answers survived into the late window on the fixed set, too few to report a percentage.
Reddit citations on the fixed set
| Assistant | Early window | Late window |
|---|---|---|
| ChatGPT | 69 of 182 (37.9 percent) | 4 of 117 (3.4 percent) |
| Perplexity | 72 of 144 (50.0 percent) | 46 of 103 (44.7 percent) |
| Gemini | 24 of 78 (30.8 percent) | 29 of 86 (33.7 percent) |
ChatGPT’s fall is a step change, and it stayed down weeks later. Gemini drifted slightly upward, and Perplexity eased off a little. We measured that ChatGPT changed, not why, and what took Reddit’s place on ChatGPT is covered separately.
Perplexity and Gemini wander: between any two windows we compare they move by roughly 5 to 15 points in either direction, which is why published reports about those two contradict each other. The sample is small, brands in competitive categories, on the questions buyers actually ask, so read it as directional.
Count only the questions asked on both dates
When you compare an engine across two dates, count only the questions asked on both dates. If the set of brands or questions changes between the dates, a change in what you asked looks exactly like a change in engine behaviour. Reddit appears far more in some categories than others, so shifting the mix of brands alone can produce an apparent collapse while the engine does nothing new. It is easy to miss, because the chart looks identical either way, and a trend line drawn over a changing question set is measuring the question set.
On anyone else’s before-and-after, hold the question set fixed. If the same questions are not on both dates, you are reading the mix, and you can stop treating that line as the engine.
