As this post is 4 years old you would do better to open a new thread in Products describing your situation.
This is actually my post from several years ago when I was with a different company! It is funny that I found it again as I was experiencing the same problem with batch sends that I did then. Although this post is old could you tell me if it is actually a viable solution? I only ask since you are a highly respected and well seasoned Marketo user.
I am trying to send to a list of roughly 4.2k in 20% random sample increments and I have created the send steps just like I previously mentioned here in my above post. After all of the emails are sent (next week sometime) then there is a short wait step and then a "Request Campaign" step so that I can move them to the next step in my Marketo program (there is a second email based on their activity with the first email).
Anyway, if this is a solution to the normal "create multiple static lists with your random sample people" answer, to batch sends, then I was just wondering so that I could update it and repost it here under my new Marketo user account. There is a screenshot below of the campaign and flow steps I am currently working on. I had to zoom out quite a bit, sorry. I'd appreciate any feedback or help. Thanks.
I'd say your setup will work somehow, but it's not really accurate and you will have a pretty big leftover cohort on day 6.
Full List 1000
20% random sample: 200
20% random sample: 160
20 random sample: 128
So that's probably not the desired outcome. The standard procedure would be to divide your full list into 5 Static Lists using the random sample flow step, and use these Static Lists in your Smart Campaigns.
Here's another idea: I just recently detected (was I blind before?) that "Random Sample" is also a Smart List filter. So could run something like this on Day 1:
On Day2 you might need need to increase the Random Sample value a little to get to 200 again, and then do the same for the next three days.
So why not do this in two steps:
That way you avoid the shrinking audience issue.