We took every ad signal you've asked about — CTR, cost-per-engagement, cost-per-3-sec-view, thumbstop, % new visits, and new-customer % — and tested each one against the only outcome that matters: does the ad acquire customers at a low CAC? Across ads and 12 months, so it's signal, not 14-day noise.
The two metrics that best separate a low-CAC ad from a high-CAC one are % new visits and new-customer % — i.e. whether the ad actually brings in new people. The pure attention metrics are weak or worse: thumbstop moves CAC only ~15%, cost-per-engagement and cost-per-3-sec-view show no reliable link at all, and CTR is actively misleading on its own. If you want to spot a winner early, watch who the ad brings in — not how many people it makes pause.
Each signal's best-third vs worst-third spend-weighted CAC, the gap between them, and the per-ad rank correlation. Ranked by how reliably the signal flags a low-CAC ad.
| Signal | Best-third CAC | Worst-third CAC | CAC gap | Rank ρ | p | Verdict |
|---|
Ads split into thirds by the signal (best / mid / worst), then the spend-weighted CAC of each group. If the signal works, the green (best) bar sits well below the red. Watch CTR break this rule.
Per individual ad, higher CTR looks like it tracks lower CAC (rank ρ = −0.34). But weight by spend and it flips: the highest-CTR third of ads carries a $101 CAC, while the lowest-CTR third sits at $74. Why? Your lowest-CTR spend is concentrated in high-intent placements (branded search, retargeting) that convert cheaply, while high-CTR broad video buys cheap clicks that convert worse.
Takeaway: CTR's meaning flips depending on ad type, so it's useless as a standalone winner-signal. Cheap clicks ≠ cheap customers.
Every signal and the CAC outcome for the 20 biggest-spend ads. Scan the % new visits and new-customer % columns against CAC — the relationship is there; the attention columns are noise.
| Ad | Type | Spend | CTR | Cost/Eng | Cost/3s | Thumb | %New | NewCust% | CAC |
|---|
% new visits is the cleanest early tell of a low-CAC ad and isn't gameable by a flashy hook. Put it on the new-ad scorecard above thumbstop and CTR.
Cost-per-3-sec-view and cost-per-engagement carry no reliable CAC signal; thumbstop is weak. Use them to diagnose a creative, never to decide scale or kill.
It flips direction by ad type. A high-CTR broad-prospecting ad is not a buy signal unless its new-customer conversion holds up.
It separates CAC best of all (−33%), but it shares "new customers" with the CAC formula, so it's partly definitional. Real, but not an independent crystal ball.