A/B Test Sample Target vs Minimum Test Duration
Compare sample-size and run-duration requirements when planning or monitoring an A/B testing service-level agreement.
A/B testing SLAs commonly set both a minimum per-group sample target and a minimum live duration. Neither requirement replaces the other: the calculator reports the lower completion value until both are satisfied.
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About A/B Test Sample Target vs Minimum Test Duration
A/B testing SLAs commonly set both a minimum per-group sample target and a minimum live duration. Neither requirement replaces the other: the calculator reports the lower completion value until both are satisfied.
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Comparisons
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Key Factors
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High-traffic experiment
The sample target is reached quickly, but the test is still early in its required live period.
| Factor | Option A: Per-group sample target | Option B: Minimum test duration | What It Means |
|---|---|---|---|
| What it measures | Collected eligible visitors in each group | Completed calendar days live | They measure different operational conditions. |
| Likely constraint | Usually met quickly with high daily traffic | Can remain incomplete despite ample traffic | A fixed duration can be the binding requirement in high-traffic tests. |
| Sensitivity to traffic changes | High | Low | A duration threshold is unaffected by visitor volume, while sample collection is not. |
| SLA completion effect | May be 100% early | May remain below 100% | Overall completion uses the lower of the two values. |
For high-traffic tests, sample collection may finish early while the duration requirement continues to control SLA completion.
Low-traffic experiment
The required live period has elapsed, but the smaller group remains below its sample target.
| Factor | Option A: Per-group sample target | Option B: Minimum test duration | What It Means |
|---|---|---|---|
| Likely constraint | Often incomplete at the minimum day count | Reached on schedule | Low traffic slows visitor accumulation. |
| Effect of a 50/50 split | Maximizes the smaller group's traffic for one variant and one control | No direct effect | Allocation changes time to sample but not elapsed days. |
| Value of extending the test | Adds visitors toward the target | Already satisfied | Additional time is useful operationally when sample collection is incomplete. |
| Recommended duration | Determines estimated days needed | Sets a floor | The longer requirement sets the planning duration. |
For low-traffic tests, the sample target usually determines how long the experiment must remain live.
Balanced vs uneven traffic allocation
The allocation choice changes how fast both groups can meet the same per-group visitor target.
| Factor | Option A: Balanced allocation | Option B: Uneven allocation | What It Means |
|---|---|---|---|
| Smaller group daily traffic | About half of eligible traffic | Below half of eligible traffic | A balanced split gives the smaller group its largest possible share for two groups. |
| Time to equal per-group target | Typically shorter | Typically longer | The lower-allocated group accumulates visitors more slowly. |
| Control exposure | Reduced relative to an uneven control-heavy split | Can preserve more control exposure | Operational goals may affect allocation choice. |
| SLA calculation | Groups are equal in the estimate | Smaller group controls sample completion | The same per-group target must be satisfied by both groups. |
| Best use | Efficiently collecting comparable group samples | Situations requiring limited variant exposure | The appropriate split depends on the experiment's operational constraints. |
A balanced split is generally faster for reaching equal per-group samples, while an uneven split requires planning around the smaller group.
Key Differences at a Glance
Sample-size requirements measure eligible visitor exposure in each group.
Minimum-duration requirements measure elapsed live-test time.
Overall SLA completion is limited by whichever requirement has less progress.
Traffic allocation affects sample collection speed but does not change elapsed days.
An uneven split makes the lower-allocated group the sample-size constraint.
How to Decide
Assumptions
- The comparison assumes one control and one variant.
- The same required sample applies to both groups.
- Eligible traffic and allocation remain broadly stable during the estimate.
- The SLA includes both a sample requirement and a minimum-duration requirement.
Related Comparisons
Frequently Asked Questions
Which matters more: sample size or test duration?
Neither always matters more. Under this SLA method, both must be met, and the less complete requirement controls the overall result.
Does a balanced traffic split always make sense?
It is generally the fastest way to reach equal per-group samples, but operational constraints may justify an uneven allocation.
Can more traffic replace a minimum duration requirement?
Not in this calculation. More traffic can complete the sample requirement sooner, but it does not reduce the entered minimum live duration.
Can a longer duration replace a missing sample target?
Only if the additional days bring the smaller group to its required sample. Time alone does not guarantee enough eligible visitors.
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