[jackgolding]

The stats went well over my head but as a web analyst I thought maybe you could have asked a simpler question. Pick a segment of your site such as visits from Google Search who landed on your homepage (most likely people who searched "gwern") which should reduce a lot of those spikes.

[gwern]

Subsetting will also increase the variance of each datapoint (consider the extreme case of picking a subset which was 0 or 1 visits per day), so is probably not a win. It's also hard to imagine what subset properly reflects all sources of traffic and so is informative about the total effect of advertising. Search queries definitely is not it.