I played around with some numbers tonight just to get a little more specifics. I was kind of surprised. Since I did my last project on running it looks like the sport has started to grow again. Here's a graph of the number of people that participated in a race during that year.
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While it is off it's peak 2017 was the 2nd highest year in history for race participants. Based on that I don't think that would be a factor for Disney
Next I looked at total finshers by race for Disney (they don't publish that for the 5k though)
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What I think's interesting is while there is a dip in half participation 10K participation is growing. It will definitely be interesting to see how big the drop off is for 2019.
I also thought it was interesting that there was a dip in the full in 2017. Since people were able to switch from the full to it I would have expected to see a small bump not a dip.