This was over 6 hours of research and number crunching ... there's still more to add, but I have to be off to work shortly. If you appreciate this, I accept shirts as donations. ^_^
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To start this off, we have to go back to when the 3k cap was in place. To extend out what the potential sales would be, I averaged the hourly sales breakdown to come up with approximately how many sales are accounted at what hour. There's a chart on every shirt's sale stats ...
I'm using seven first place shirts that sold out their 3k on debut day:
Week 180, No One Left to Play With in 12:56, which is approximately 68% of a day's sales.
Week 181, Gardening at Night in 18:07, which is approximately 84% ...
Week 185, Acquired Taste in 8:10, which is approximately 42% ...
Week 186, Cooler Than You in 11:07, which is approximately 60% ...
Week 187, Unstealthiest Ninja in 10:43, which is approximately 58% ...
Week 188, They call me… Tim in 11:43, which is approximately 62% ...
Week 191, Tea! in 18:45, which is approximately 86% ...
Without a cap, theoretically they would have sold 4412, 3571, 7143, 5000, 5172, 4838, and 3488 on debut day, respectively, totalling 12624 sales lost on debut day for those seven. Ultimately, they sold 3440, 3192, 4148, 3376, 3567, 3414, and 3190 through Monday. Presuming the following three days were the return customers, only 440 of 1412, 192 of 571, 1148 of 4143, 376 of 2000, 567 of 2172, 414 of 1848, and 190 of 488 ended up coming back, which averages out to 28.39%.
BUT it's not that straight forward, as even with the higher cap, shirts still sold over the weekend, which means that not all of those subsequent sales were due to the sold out sign.
With that in mind, here are five first place shirts that sold more than 3k but less than 5k on debut day:
Week 199, They're Watching, 3805 debut, 3937 through Monday.
Week 200, In the Library With the Wrench, 4309 debut, 4545 through Monday.
Week 202, Funny Things are Everywhere, 3841 debut, 4003 through Monday.
Week 204, Scissors Paper Rock, 4599 debut, 4863 through Monday.
Week 208, The Three Classical States of Matter, 4784 debut, 5019 through Monday.
Removing 3000 sales to establish the same basis, their Saturday/Sunday/Monday sales average 14.26% of their >3000 sales. Applying the numbers to five more >3000 shirts and their next three day's sales ...
Week 213, Cell Division, 3603 debut, 3820 through Monday. 217/820 = 26.46%
Week 214, Haunted Housework, 3991 debut, 4191 through Monday. 200/1191 = 16.79%
Week 216, Always Leave a Note, 4368 debut, 4539 through Sunday. 171/1368 = 11.11%
Week 218, Click or Treat, 4794 debut, 5105 through Friday. 311/2105 = 14.77%
Week 219, Star Map, 3680 debut, 3798 through Monday. 128/798 = 14.79%
10 shirt average, 15.53% - A difference of 12.87%. In other words, roughly 1/8 comes back to buy a shirt at $15 when faced with the sold out sign.
Last week was the first week of the 6k cap. The first shirt to push it is This Was a Triumph, 5468 debut, 5687 through Monday. 219/2687 = 8.15%
I will wait until tonight to see what Family Breakfast closes out to (and see if the math corresponds), but so far, the numbers say that indeed, the higher cap does have a negative effect on the artist's subsequent sales.