The Inbox Clearing Index, where are you?

The Inbox Clearing Index, where are you?

I wanted to tell you about an inbox clearing study in which we took a peek into thousands of Outlook inboxes of knowledge workers around the world. The study, in collaboration with my colleague Dr. Gilad Ravid from Ben-Gurion University of the Negev, examined a dataset that included millions of datapoints which describe, on an hourly basis, the inboxes of the Outlook users: the number of messages in the inbox, number of read messages, number of messages sent, number of responses sent, average response time, and number of unread messages. What did we find?

  • People are busy: On average, our users had 1551 messages in their inbox, of which 345 were unread messages. They received 57 messages a day, sent 14, and removed (erased/filed away) 58. Their average response time was about 29 hours. These, of course, are only averages, and there is a lot of variation between users. For example, the 10th percentile of our users received on average only 10 messages a day. The 90th percentile received 109. Remember: these averages are not averages of a typical workday, and also include weekends, holidays and vacations.
  • Inboxes are loaded. Users are overloaded: The only time the average inbox in our study shrinks in size is between 1am and 9am. From nine in the morning, the inbox size is constantly rising.


inbox clearing index knowmail 1

Figure 1. Average levels of inbox activity, by (local) time of day. Received per hour: filled circle, sent per hour: x, end of day inbox size: asterisk, end of day unread: empty box.

  • People are very different from each other: Unlike the classic division of users into “filers” (constantly file and organize their inbox), “pilers” (never organize the inbox) and “spring cleaners” (clear their inboxes several times a year), we see behavior that is all over the spectrum between pure pilers and pure filers. In fact, we developed an index (inbox clearing index – ICI) to measure the correlation between the number of emails a user receives every day, and the number they clear from their inbox. We show that when we measure the ICI of our user population, it is quite evenly distributed.
  • Inbox clearing is interesting: A close inspection of users and their inbox clearing behavior shows that those who have a higher ICI receive more messages a day than those who have a lower ICI, send more messages, and reply to more messages. Furthermore, those with a higher ICI index have fewer messages in their inbox, fewer unread messages, and shorter response times.
  • Averages are only averages: Although inbox activity continues throughout all hours of the day, all days of the week, and every day of the year, we see significant fluctuations throughout the 24-hour day (with a dip in activity during the night, outside typical business hours and even around lunch time), and throughout the week (with a dip in activity during the weekend). We examined only 250 days of activity, so we did not look at fluctuations during different periods of the year.

inbox clearing index knowmail 2

Figure 2.  Average levels of inbox activity, by day of the week. Legend as in Figure 1.

In summary, our study shows how thousands of users around the world deal with the constant inflow of hundreds of messages a week and the outflow of many dozens of messages. It shows how messages accumulate in the inbox around the clock and throughout the workweek and the weekend, and are then read, responded to, filed away, or erased. It demonstrates that despite the increasing number of media available to us, e-mail is still a key communication medium in the workplace, and that effectively managing the inbox is still one of the key challenges of the contemporary knowledge worker.

The link to the study. To receive an author’s copy of the study, with all the graphics and tables, please email the author.


Yoram Kalman
Yoram Kalman

Yoram Kalman researches the impact of digital technologies on individuals, organizations and society. He is a senior lecturer at the Open University of Israel, and studies computer-mediated communication (CMC), human-computer interaction (HCI) and information systems (IS).

Yoram Kalman

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  1. I wonder at “The only time the average inbox in our study shrinks in size is between 1am and 9am”. Does it mean users are clearing mail after 1am? Or at 8am?