The only real solution I've noticed, and please correct me if I'm wrong, is to aggressively recruit minorities. It was really late in the game, but one person did manually contact 60 women to see if any would be interested in speaking at this particular conference, but not a single one even responded to the email, let alone offered to come and speak. Perhaps the results would be different had that happened further in advance, but we don't know.
It's worth noting that this conference has a large percentage of attendees who are women. Women just aren't represented in the speaker list. Over the 4 years that it's happened, there have been a handful of women speakers. This may hold true for other events.
I'm hoping this form will help get some insight from minorities and hopefully get some ideas on how we can fix it.
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