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At the beginnings of this site, when we started measuring ourselves, user @EnergyNumbers informed in a comment here in meta, that the metric "Visits per Day" shown in Area 51, is a median, not an arithmetic average. And we have since then used this information in interpreting this metric.

But is it? Starting yesterday we had one more "spike question", this one , which currently runs at $5k$ visits. And the Area 51 metric "Visits per day" shows a value of $1,599$, while 29 days ago it showed $1,404$. Although not impossible, this is not the behavior expected from a median, but from a mean... or from a short-sighted median.

I used the access I have to site-analytics, and compared with previous meta-posts of mine where I have captured the metric at various time-points. Results: it appears that the metric Visits-per-Day is a 7-day rolling metric. But sometimes it looks like it is a median (with the median calculated as the average of the middle value, and the value before it), while in others, it appears to be the arithmetic average of this seven-day interval.

So can somebody from SE clarify this please?
We' re economists, meaning that
a) we like very much using metrics
and
b) we abhor using metrics that we don't know exactly how they are constructed.

BY THE WAY:

Our 30-day rolling average for Visits-per-Day grows very happily: In October 1, 2015 it was $911$. In November 1, it was $1,340$ (+ $47\%$)

The 90-day rolling average was $644$ in October 1, and $933$ in November 1 (+ $45\%$).

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  • $\begingroup$ Specifically, it's a two-week median (did I say that in the comment you saw?) $\endgroup$ – EnergyNumbers Nov 18 '15 at 9:13
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This seems relevant: What's the difference between stackexchange.com/sites traffic numbers and Area 51 traffic numbers?

A quote from that answer:

stackexchange.com/sites was just updated today to use a new formula for traffic. Area 51 is still using the old formula, but will be updated soon to use the new formula.

New Formula: Average # of Visits each day for the past 14 days as recorded by Google Analytics

Old Formula: Total # Question Views / Total # of Days as recorded in the API

The old formula wasn't very good at all because (1) it only counts question views and (2) it's averaged for all time, so it doesn't reflect where the site is now. So sites that have grown were showing lower numbers than they should, and sites that have shrunk were showing higher numbers than they actually have now.

Update: Area 51 is now using the new formula

They don't specify what kind of average, but perhaps you can infer that from the data you have?

If any residual confusion remains, I suggest posting a question at meta.stackexchange.com where the network admins seem quite open about the details of their formula.

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  • $\begingroup$ Moderators and the 35k equivalent privilege for betas have access to Google Analytics, just reference. $\endgroup$ – Braiam Nov 26 '15 at 20:17
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After the link provided by @Ubiquitous, I tested the $14$-day rolling metric... and it appears that "Visits per Day" is 14-day rolling median, and not an average as the SE posts says.

Specifically, the current 14-day rolling mean would give a value of $1,512$, while the 14-day rolling median gives $1,599$ (as the average of the two middle values).

I also notified the OP in meta.SE, and he has corrected his post accordingly.

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    $\begingroup$ Formally, the median is an average. As is the mean and the mode. Colloquially, you're right: average is typically used to denote the mean. $\endgroup$ – EnergyNumbers Nov 18 '15 at 9:12

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