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Halo Effect
Halo effect is the incremental lift in conversions, search volume, or direct traffic on one channel caused by exposure on a separate channel, typically a rise in branded search or direct visits following TV, video, or display activity. It is isolated by comparing the receiving channel's performance during and after exposure against a baseline or control group with no exposure, so the lift is credited to the sending channel instead of being double counted by the receiving one.
Formula
Worked example
A home goods retailer in the Netherlands runs a TV flight worth 800 GRPs over 4 weeks. Branded search conversions averaged 3,200 per week in the 4 weeks before the flight (baseline). During the flight, branded search conversions averaged 4,000 per week. Halo lift = (4,000 − 3,200) / 3,200 x 100 = 25%, meaning 800 of the 4,000 weekly branded search conversions, 3,200 over the full flight, are incremental and attributable to the TV exposure rather than to search activity on its own.
How it is used
Planners use halo lift to justify awareness-channel budgets that look weak on last-click metrics but drive volume elsewhere, most often TV or video lifting branded search and direct traffic. It is measured with geo holdout tests, matched market pairs, or pre/post baselines adjusted for seasonality and promotions. The common mistake is reading a raw pre/post comparison as halo without controlling for seasonality, competitor activity, or concurrent promotions, which inflates the attributed lift.
The common mistake
Measure halo against a matched control group or seasonally adjusted baseline, not a raw pre/post comparison, or the lift will be overstated and partly double-counted with conversions the receiving channel already claims.