Anything pixel-based, no. Brand lift, no. Causal lift, search lift, and halo effect, yes. Footfall, sometimes, within a window.
What cannot be added later
Web attribution: the pixel captures from its install date forward, and traffic during the gap is not recoverable because it was never recorded. The one exception is a pixel from the same provider we work with that was already in place, in which case that data can sometimes be purchased, subject to a third party agreement.
The Shopify integration: same constraint, same reason.
Brand lift: the survey needs exposures captured while the campaign is live, and needs its brief ready early enough to field. Added late, it measures a fraction of the flight or nothing at all.
What can still be run after the fact
Causal lift: it runs on historical event data, so it can be commissioned after the campaign ends.
Search lift: built from search volume data, with no setup, so it runs retroactively.
Halo effect: it reads your connected accounts' history, so it can be run after the fact provided the accounts can be authorized and there was activity to read.
What can sometimes be added
Footfall attribution can often be added after launch, within roughly a six month window, because exposure and location data do not depend on anything installed on your side. Ask, because it is possible more often than customers expect, and it is frequently the difference between a campaign that gets a measurement story and one that does not.
If your campaign is already live
What remains for most campaigns is causal lift plus search lift plus, often, footfall. That is a real measurement package. It is not the one that was planned, and it is considerably better than concluding that the campaign is unmeasurable.
Common misconceptions
Myth: Measurement can be applied retroactively to a campaign in flight.
Reality: Not for anything pixel-based. The pixel only captures from install forward.
Myth: Nothing can be measured once a campaign has launched.
Reality: Causal lift, search lift, and halo effect all run retroactively, and footfall often can too.