Why setting the order of treatments matters

With Split, rules are evaluated from the top down. 

1. Individually target - users you explicitly assign to a treatment
2. Limit exposure - If set to less than 100%, the excluded users are given the default treatment you specify in the treatments list.
3. Targeting rules - specific subsets of users targeted by attributes
4. The final rule, which is labeled as "Serve" or "Distribute treatments as follows" in the UI - users not individually targeted and not targeted by a targeting rule. This is a "catch-all" which assigns treatments for the remaining population not previously excluded or targeted.

As with rules, the order of treatments matters.  For example, if you have 3 (or more) treatments you’ll want to think through order based on how you want things to change as you change percentages, particularly if you want to maintain a consistent user experience, as described here.

Let's use changing the default rule as an example, with status_quo, treatment1, treatment2 set to 33/33/34.  You want to move everyone out of status_quo and set treatment2 to 67%.

If you had your treatments in the above order (status_quo, treatment1, treatment2 ), setting status_quo to 0% and treatment2 to 67% would cause the users from status_quo to move to treatment1 and the users from treatment1 to move to treatment2.  Clearly, this may not be the right experience for your users.

If, however, you have status_quo in the middle of the order (treatment1, status_quo, treatment2 ), then when you set status_quo to 0% and treatment2 to 67% this does not impact the users in treatment1.  This would also be true if you set treatment1 to 66% and status_quo to 0%.  The 34% originally assigned to treatment2 would remain consistent.  See how this can apply to rolling out an experiment below.

Let's say you have more than three treatments.  In this case, the status_quo was the safe treatment, and if any of the treatments were found to have a bad experience the plan is to move those users to status_quo.  If we decide treatment2 was bad and we try to give that 20% to status_quo by setting treatment2 to zero and increasing status_quo to 40%, we will actually cause an undesired shift: users in treatment1 would get status_quo as well since the first 40 buckets, which had been distributed between status_quo and treatment1, now all get status_quo. Buckets 40-59, which had been getting treatment2 would then get treatment1. Users who were getting treatment3 and treatment4 are unaffected.

One way to 'move' users between multivariate treatments while only removing users from treatment2 is to take advantage of Dynamic Configuration, assuming you can represent the differences between treatments using a set of configured values.  In this case, you'd set the configuration of treatment2 to the status_quo settings, which would give them the status_quo experience.

Limiting traffic

Limit exposure allows you to exclude a percentage of users from your rules. For example, you can set exposure to 20% and then set the "Distribute treatments as follows" setting to 0/50/50 for status_quo, treatment1, and treatment2.  Effectively, each of the two 50/50 treatments gets 10% of traffic.  The advantage of this approach is that as you add traffic to the rules by increasing the limit exposure setting, no users switch from, in our example, treatment1 to treatment2.  They move from status_quo (set as the default treatment) to one of the two treatments.

This is also useful if you have a number of complex rules and you want to increase the participation across all the rules at once. 

Including traffic for experimentation

But what if you want to see all of the impressions on the Metrics impact tab? Even if you're not going to include the unallocated status_quo users in the analysis. Again, order matters if you want to make sure adding traffic doesn't move users that are in the experiment between treatments.  In this case, you'd order the excluded users to be between the two treatments.

As you roll-out more users, by decreasing the percentage in status_quo and increasing each of the other treatments, existing users in those two treatments will remain in the same treatment they were in previously.

Individually target

Individually targeted evaluations are done in the order of the treatments. Consider a situation where user 12345 is individually targeted for status_quo. Even if user 12345 is in the beta-users or Employee segments individually targeted for treatment1, they will still get status_quo, because that's evaluated first.
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