How does a change in daypart affect TV performance?

Per the above chart showing typical visits by hour of day, daytime delivers the highest level of Internet traffic.

C3 calculates lift by comparing visits from organic, paid brand search, and SEO in the same geographies for which the TV spot aired, against the same window.  Let's say it's a 5 minute window and the spot airs Wednesday 2.07pm.  C3 averages the visits from the same three buckets of traffic from when the spot aired from 2.07pm through 2.12pm in the same geographies, for the 8 weeks of the prior Wednesdays 2.07pm through 2.12pm.

The baseline is thus a prior 8 week average (the baseline timeframe can be changed).  Let's first describe how lift is calculated.  Let's say the baseline is 100 visits from that avg same 5 minute period in the same exact geographies, for the same three buckets of traffic (organic, paid brand search, and SEO).  If after the spot aired, the visits for the same geographies and same buckets of traffic amounted to 150 visits, then the lift would be 50%.  If the traffic was 102, it would be 2%, etc.  If the traffic was 90 visits, there would be no lift.

Let's use the example of 50% lift:  C3 decrements any attributed value for organic, paid brand search, and SEO appearing in that 5 minute window by 50%, and then increments the specific TV spot's attributed value by the 50% that was decremented from organic, paid brand search, and SEO.  If there is spot overlap, a station weighted average distributes credit based on the HH ratings of the spots which air/overlap in the same attributed window (note long TV attribution windows can cloud actual performance even more when you understand the station weighting algorithm).

How would a shift in day part affect this? 

C3 deploys the same methodology throughout, but Internet traffic in terms of absolute volume is much higher in the daytime.  So if a spot airs during the daytime, there is obviously traffic coming to the site from organic, paid brand search, and SEO which has nothing to do with TV--regular course of business.  As a result, TV spots that air M-F during the daytime have a harder hill to climb in order to create lift versus hours outside peak M-F daytime.

So a shift of more spend into daytime will, by the math, will likely show a lower ROI.  A shift of more spend into daytime, with an abnormally long window will be even harder to generate lift and show ROI.

Alternatively, a shift of TV spend out of daytime will likely show a higher ROI; and a change in shortening the window combined with more TV dollars to the 5pm through the night hours...will show more lift.  But lift is not ROI, just the precursor: primetime is more expensive, so even though it might show more lift, depending on the degree of increased cost and response, it could show lower ROI because of the higher CPM's.