13
weeks of the year from year to year, I inserted some “fake” weeks to re-scale the year so that all
holidays fall on the same week across all years (1999-2003). Without this scaling, I may fail to
capture changes in underlying market size associated with specific holiday weekends; this
generates a total of 56 weeks in a year (Einav, 2004).
I define videos that are no more than two weeks old (
t - r
j
= 0 or 1) as “new”, since
common industry perception maintains that the popularity of a video falls dramatically after first
two weeks of its release. My results are not sensitive to alternative definitions of newness (i.e.,
released this past week, released within the past three weeks) or if I allow decay to vary by
season. For genre classification, I use information from the IMDB database which lists up to six
different genre classifications for most movies.
11
For example, IMDB classifies “Shrek” as both
a comedy and family movie. I initially assign each movie to its first genre classification. Then I
group similar IMDB genres into broader categories by combining the action, adventure, and
fantasy categories and by combining crime, thriller, and mystery movies under the label of
suspense.
12
Comedies represent the most popular category and account for nearly a third of all
movies. The horror category contains the fewest number of titles (40 movies) and only represent
6% of movies in the sample.
For the 2SLS regression, I cluster the standard errors at the movie level to take into
account any serial correlation in revenues across weeks. I include a constant term in my
weekly share towards the end of the year; this would work against finding a result of higher underlying demand in
the holiday season.
11
I obtain similar results when I use genre classifications from the Variety Weekly Box Office Charts. For about 30
out of 620 movies in such small categories as Western, Musical, or Science Fiction with no corresponding genre
nest, I used the secondary classification from IMDB.
12
The western, romance, and science fiction categories encompassed only a handful of movies in my sample, so I
re-categorized each of these movies under its secondary genre classification. “Real Cancun” was the only video
listed as a Documentary with no alternative genres, and I re-classified this as a Drama. In particular, most romance
movies were either of the romance-comedy format or romance-drama format, and only one western movie and a
handful of science fiction movies appeared in sample. The family category includes all films that appeal to both
children and non-children audiences; that is, I included movies in the family category if they were listed as a family
or animated movie under any of their alternative IMDB classifications.