When I was the Assistant Director at The Skyscraper Museum (NYC), we did the
following:

1.  Encouraged each visitor to sign the guest register (we called them 
    "signees").
2.  Entered the "signee" information into a spreadsheet program,
    including the date of attendance and U.S. home region (e.g., NYC
    borough name, NYC Metro, Northeast, Midwest, etc.), and/or world 
    region if outside the U.S. (e.g., Western Europe, Canada, etc.).  
3.  Took a "clicker" count of everybody that visited during a
    one-week period, recording the results for each day of the week.  
4.  Created a formula that considered the relationship between the
    "clicker" and "signee" counts for each "clicker" day.  We then used
    the formula to estimate "nonclicker" days.  For example, on 
    Thursdays, if the difference between the "clicker" and 
    "signee" rate was 70%, we would assume that the number of attendees
    for a "nonclicker" Thursday was 70% higher than the number of 
    "signees" on that day.  Given that we had recorded the information 
    geographically as well, we applied the same formula to each region.
5.  Periodically, to update our percentages, and to track changes in 
    visitorship, we would do a another cycle of "clicker" days.

Short of counting every single person every single day, which we couldn't do
because museum entrance was free and it was (still is) a small museum, I
believed this was the next best system.

Michael Radice
The Union Institute/New York University

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