Study: Why seasonality adjustments keep failing advertisers on Black Friday

A three-year analysis of up to 6,000 advertisers shows that using Google’s seasonality bid adjustments during Black Friday and Cyber Monday (BFCM) consistently harms efficiency — even though the platforms keep recommending them.

The big picture. Smart Bidding’s models are designed to anticipate predictable retail spikes. Optmyzr’s analysis of tens of billions of impressions across 2022–2024 found that advertisers who skipped seasonality adjustments consistently performed better on efficiency metrics.

Without adjustments, Smart Bidding:

• Identified the BFCM conversion lift on its own
• Raised bids rationally
• Kept ROAS stable — even improving it in 2024

With adjustments: CPCs surged much faster than actual conversion rates, eroding efficiency.

Reality check: Google doesn’t need your “heads up”. Seasonality adjustments tell Google: “Expect conversion rate to rise by X% — bid that much harder.”

But Smart Bidding takes this literally. If you guess wrong — and almost everyone does — the system overshoots.

Example:

• You predict +50% CVR lift
• Real lift is +40%
• You’ve effectively overbid by 7.1%

During BFCM’s high volumes, even small gaps get expensive fast.

The data: 3 years of the same story

1. Smart Bidding already catches the CVR spike

• 2022: +17.5%
• 2023: +11.9%
• 2024: +7.5%

No help needed.

2. CPC inflation doubles with adjustments

Across all years, CPCs jumped about 2x higher when advertisers used a seasonal adjustment.

3. ROAS drops significantly

Advertisers who trusted Smart Bidding saw stable or improved ROAS.
Those who intervened saw double-digit drops.

The one exception: “Volume at all costs”. If your marching order is pure revenue growth — margins be damned — seasonality adjustments deliver.

Revenue lift was consistently higher when advertisers used adjustments:

• 2022: +50.5% vs. +25.0%
• 2023: +52.8% vs. +30.3%
• 2024: +39.9% vs. +33.8%

Efficiency tanks, but volume pops.

When seasonality adjustments actually make sense. Use them when Google doesn’t have advance signal — not for global events like BFCM.

Good for:

• One-off flash sales
• Email-only promotions
• Surprise clearance events
• Niche seasonal spikes

Not good for:

• Black Friday
• Cyber Monday
• Christmas
• Valentine’s Day
• Any event with years of predictable history

Why we care. Google already knows it’s Black Friday. Smart Bidding is trained on years of BFCM data and reliably detects the conversion-rate spike on its own. Overriding the system often causes aggressive overbidding, inflated CPCs, and lower ROAS – meaning many marketers are unknowingly burning budget during the most competitive week of the year.

By understanding when Smart Bidding already has the signal, advertisers can avoid costly mistakes, protect efficiency, and reserve seasonality adjustments for moments where they actually create value.

Bottom line. Smart Bidding handles major retail holidays better than most advertisers. Seasonality adjustments usually introduce more volatility than value. Skip seasonality adjustments for predictable retail peaks. Save them for brand-specific events Google can’t foresee.

Smart move: Trust the algorithm — and use guardrails, not overrides. Tools like anomaly alerts, pacing monitors, and bid caps provide control without fighting Smart Bidding’s core models.

Dig Deeper. Do Seasonality Adjustments Actually Help During BFCM? A 3-Year Study Says No.