The universal published answer is "a competitor entered the auction," which is unfalsifiable and useless. Before you accept it, separate a genuinely more expensive auction from a shifted traffic mix. They look identical in the aggregate number and they have opposite fixes.

On this page

• Why is average cost per click a misleading number?
• How do I separate a more expensive auction from a shifted traffic mix?
• How often is a cost-per-click rise just mix rather than competition?
• What should I check if the auction really did get more expensive?
• What measurement artifacts fake a cost-per-click rise?

Why is average cost per click a misleading number?

Average CPC is a spend-weighted average across segments that cost wildly different amounts. If delivery shifts toward expensive segments — mobile, a pricier geography, broader match types, non-brand instead of brand — your average CPC rises without a single auction getting more expensive.

Nothing got worse. The composition changed.

How do I separate a more expensive auction from a shifted traffic mix?

Split spend into segments, then compute the counterfactual: your current efficiency at last period's mix.

• blended = Σ(share_i × cpc_i)
• Efficiency effect = (current efficiency at old mix) − old actual
• Mix effect = current actual − (current efficiency at old mix)

Hold the mix constant, recompute, and see what is left. What remains is the real auction move.

Segments worth holding: keyword or keyword theme, match type, device, geography, network, and brand versus non-brand.

How often is a cost-per-click rise just mix rather than competition?

More often than people assume. Running this on a blended-performance claim in a different context, 103% of the gap turned out to be mix and the efficiency effect was slightly negative. The headline number said one thing; holding composition constant said the opposite.

The same logic runs in reverse, and is worth knowing if anyone ever tells you they improved your account: check whether they just shifted budget toward brand. Brand converts at 20× for whoever is running it, so moving budget there lifts every blended number with no skill involved.

What should I check if the auction really did get more expensive?

Then it is a real auction move, and the honest checks are:

• Impression share lost to rank rising while budget-lost holds steady. That's competitive.
• Auction insights, read for overlap and position change, not for the vanity column.
• Query mix within a match type — broad match drifting into more expensive intent territory counts as a real cost rise, not a mix artifact, because you did not choose it.
• Seasonality. Compare against the same weeks last year, not against last month.

What measurement artifacts fake a cost-per-click rise?

Month boundaries. Google throttles delivery near the monthly ceiling of daily budget × 30.4, then resumes on the 1st. Windows spanning that boundary are unreliable in both directions.

Composition drift in what you're counting. On a longer series, check whether the conversion actions being counted changed across the span. On one account an extra action was folded into the counted metric for exactly seven months, and every ratio built on it for that period was measuring something different from the months either side.

Related questions

My Google Ads conversions dropped and I didn't change anything. Is it broken, or is the data still filling in?

Before you diagnose anything, check whether the window you are reading is old enough to be complete. If any day in it is less than 4 days old, the number is wrong and there is nothing to diagnose yet. Read the full answer.

My Demand Gen campaign's views exploded and sales collapsed, and I didn't change anything — what happened?

Check the channel controls before you look at creative, audience or bidding. Google has been folding standalone Display into Demand Gen, so a campaign can start buying open-web inventory without anyone having chosen that. Read the full answer.

My Local Services Ads leads suddenly stopped and I didn't change anything — what do I check first?

Check your Google Verified status and your budget pacing before anything else. A profile suspension and a paced-out budget both produce a total stop, and everything else produces a decline rather than a stop. Read the full answer.

Written by Usama Khan, who runs Google Ads for e-commerce brands and home-services companies at AdventurAds. Figures are from accounts under active management and are anonymised. Last reviewed 2026-08-26.