The short answer
A stockout can change what happens after an advertising click. Customers may leave, choose a substitute, or buy later. Your ROAS report shows recorded revenue under the observed conditions; it does not reveal the sales that would have happened if every product had remained available.
Match availability with delivery dates
Compare product availability timestamps with ad spend and landing-page destinations. Distinguish a full stockout from unavailable sizes, colors, or locations. Check whether an ad still promises an offer that visitors cannot purchase. Record when the destination or offer was changed.
Read a before-and-after comparison carefully
Illustrative example: an in-stock period has $500 spend and $2,000 revenue, or 4× ROAS. A later period with a stockout has $500 spend and $1,000 revenue, or 2×. The observed revenue difference is $1,000. It is not a defensible estimate of lost sales without accounting for other changes and a credible comparison.
Traffic quality, promotions, competition, weekday mix, and conversion delay could also differ. Label the availability issue as a possible explanation until the evidence supports a stronger conclusion.
Substitutions change economics
Suppose, illustratively, a substitute sells for $100 but has $70 of variable costs, while the usual item sells for $100 with $50 of costs. Both generate the same revenue numerator, but contribution before advertising is $30 versus $50. A stable ROAS can therefore hide a weaker product mix.
Make the next decision explicit
Depending on actual availability, consider aligning the destination with an available relevant item, correcting the offer, or reducing delivery to unavailable products. Evaluate each option against expected contribution and customer experience. Do not assume a replacement item has the same conversion rate.
After inventory returns, allow time for measurement to mature and compare similar periods. Keep the stockout dates in the report so future target-setting does not treat a constrained selling period as normal demand.
Annotate availability constraints
- Gather: Match product availability timestamps to spend, destinations and substitute product costs.
- Produce: Report observed performance and any product-mix shift before deciding on delivery changes.
Decision boundary: The before-and-after revenue gap is not automatically the amount of lost sales.
Next reading
Examples are illustrative, not client results or benchmarks. The calculations use the inputs shown; real-world decisions depend on your records and assumptions. Report a correction.