All evidence I've seen suggests you either get one right when they're first sent out, or you don't get one at all. The EX raid scheduled for Saturday in my area only had two people say they got invitations that first day, and no one else has said anything about getting one since then. I also haven't seen a single normal raid at that gym all week, so no chance to go raid there to try to trigger an invite.
GamePress
Ex Raid Invites
Do they ever send out more passes to the same event in waves?
A group of 9 always have hit the same sprint store (in groups of 2-3) but hit the same Raids. 1 Suicine, 1 Golem, 1 Totodile. always going in 3+ at once.
Only 1 of the 9 got an invite. For 11/26
Is it possible others will? Or once a set goes out, thats it for 11/26
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Answers
I think the concept is ok but they have to publish how the passes gets handed out so that people know what to expect (and know what to do to help their chances). I do understand if they don't publish this while they are testing different setups (although it, quite frankly, wouldn't hurt them to be more open about that too).
This would be my suggestion:
All trainers who have completed a raid at X over the last Y days have an equal chance to get a pass (more raids doesn't help).
This way you know to spread out your raiding over the Y day period (and rotate back after Y days are up). They could even add a little tag to your gym list where it shows if you are "in the running" at a gym or not.
There are a few more additions I would suggest.
1) Give some priority to those who have not done any ex-raids. If, for example, they are going to hand out 20 passes for a raid, they could hold back 4 while they do the random assignment. If it turns out that less than 4 of the 16 passes handed out went to "newbies" they could now hand out 1-4 passes from the extra pile to the newbies.
2) If they are doing multiple ex-raids on the same day, don't let any account get multiple invites (it has happened in my area at least once).
3) Be clear about how gyms are selected (right now all trainers are making sure they hit sponsored gyms in their area since they've had the best chance of hosting ex-raids since the testing started. That is almost ok but needs to be clarified. I say "almost" because it punishes those with "child" accounts that cannot see sponsored content. They still spend money on their accounts!).
