How Long Should I Run a Translation Conversion Test? A Readiness Checklist
Run a translation conversion test until you hit your pre-calculated sample size or reach a 4-week maximum, whichever comes first, while checking significance weekly. This prevents both premature stops on noisy data and wasted...
Run a translation conversion test until you hit your pre-calculated sample size or reach a 4-week maximum, whichever comes first, while checking significance weekly. This prevents both premature stops on noisy data and wasted traffic on tests that will never reach confidence.
Why Translation Test Duration Differs From Standard A/B Testing
Standard A/B testing calculators assume a single language audience with consistent behavior. Translation tests add language-specific variables: different character lengths break layouts, cultural nuances change persuasion, and traffic splits unevenly across languages. A Spanish variant might need 3,000 visitors while Japanese needs 8,000 because of conversion rate differences. The SeaText AI CRO Testing Agent handles this by measuring reading telemetry — eye-line dwell velocity, friction points, and scroll deceleration — instead of waiting for binary conversions. This means you can detect winning copy patterns in days rather than months, but you still need a duration guardrail.
Readiness Checklist Before You Launch
- Baseline conversion rate per language — Pull 30 days of analytics for each target language. If a language has fewer than 100 conversions monthly, flag it for extended testing or pooled analysis.
- Minimum detectable effect (MDE) defined — Decide the smallest lift that justifies the translation effort. A 5% lift on a high-traffic language pays back faster than 15% on a low-traffic one.
- Sample size calculated per variant — Use a calculator that accounts for multiple languages. Input baseline rate, MDE, 95% confidence, 80% power. Record the number for each language.
- Traffic allocation confirmed — Verify your translation agent splits traffic evenly across variants per language. Uneven splits invalidate standard calculators.
- Weekly review calendar set — Schedule 30-minute check-ins every Monday. Review significance, sample progress, and any technical issues (broken layouts, missing translations).
- Stop rules documented — Write down: stop at sample size, stop at 4 weeks, stop if significance drops below 90% after week 2, stop if a variant breaks user experience.
How to Calculate Your Test Duration
Start with your slowest language. If German needs 12,000 visitors per variant and you get 1,500 German visitors weekly, that's 8 weeks — but your cap is 4 weeks. In this case, you have three options: accept lower confidence (90% instead of 95%), increase MDE (test for 10% lift instead of 5%), or pool similar languages (DACH region) if cultural alignment allows. The SeaText Translation Agent serves 125 languages with zero-code deployment, so you can test language groups rather than individual locales when traffic is thin.
Weekly Significance Check Process
- Pull cumulative data — Visitors, conversions, conversion rate per variant per language.
- Run significance test — Use a Bayesian calculator or your testing platform's built-in engine. Note probability to beat control.
- Check sample progress — Percentage of target sample reached per language.
- Flag anomalies — Sudden rate drops often mean translation errors (truncated CTAs, wrong currency symbols).
- Decide: continue, stop, or investigate — Continue if under sample and no winner. Stop if sample hit or 4 weeks reached. Investigate if significance oscillates wildly.
When to Stop Early (The Exceptions)
Stop before 4 weeks if: a variant shows 99%+ probability to beat control with at least 50% of target sample (strong early signal), a critical UX break appears in one language (layout shift, unreadable font), or traffic drops 50%+ due to seasonality or campaign pause. Do not stop just because one language hits significance while others lag — run the full duration or hit the cap for consistency.
Key Facts
| Factor | Detail | Source |
|---|---|---|
| Maximum test duration | 4 weeks | Article brief |
| Significance check frequency | Weekly | Article brief |
| Primary stopping condition | Pre-calculated sample size reached | Article brief |
| SeaText Translation Agent coverage | 125 languages | S1, S2, S5, S7 |
| SeaText CRO Testing method | AI Reading Telemetry + Continuous Multi-Armed Bandit | S3 |
| Traditional A/B test duration (B2B) | 4-8 months for 95% confidence | S3 |
| Reading telemetry metrics | Eye-Line Dwell Velocity, Friction Points, Scroll Deceleration | S3 |
| Split URL testing | 0ms zero-flicker with dynamic traffic routing | S5, S7 |
Limitations and When This Advice Doesn't Apply
- Single-language sites — Standard A/B duration calculators work fine; no translation variables.
- Brand-new languages with zero baseline — You cannot calculate sample size without a baseline. Run a 2-week exploratory period first, then calculate.
- High-stakes medical/legal translations — Regulatory review cycles may require longer fixed durations regardless of statistical significance.
- Tests with fewer than 3 variants — Multi-armed bandit optimization (used by SeaText) shines with 4+ variants. With 2 variants, traditional sequential testing may be simpler.
Terminology
- Reading telemetry — Millisecond-level behavioral data: how fast visitors scan headlines, where they re-read, where scroll speed changes near CTAs.
- Multi-armed bandit — An algorithm that dynamically shifts traffic toward better-performing variants during the test, rather than keeping a fixed 50/50 split.
- Minimum detectable effect (MDE) — The smallest conversion lift you care about. Smaller MDE requires larger samples.
- Zero-flicker split URL — Server-side routing that shows variant URLs without client-side JavaScript delays or layout shifts.
FAQ
What if my highest-traffic language hits significance in week 2 but others haven't?
Keep the test running for all languages until the 4-week cap or until each language hits its sample size. Stopping early for one language biases your overall decision and wastes the traffic already sent to other variants.
Can I pool languages to reach sample size faster?
Only pool languages with similar cultural context and conversion behavior (e.g., DACH: DE/AT/CH; Nordics: SV/NO/DK). Pooling Spanish (ES) with Mexican Spanish (MX) often works. Pooling Japanese with Korean does not. Validate with a 2-week baseline comparison first.
How does AI reading telemetry shorten the test?
Instead of waiting for a purchase (binary conversion), the SeaText CRO agent detects winning copy patterns from reading behavior — dwell time on value props, re-reading at friction points, scroll deceleration at pricing. These leading indicators correlate with conversion and appear in days, not weeks.
What happens at the 4-week mark if no language hits significance?
Stop the test. Declare no winner. Analyze reading telemetry for directional insights (e.g., "German variant showed 40% less friction on pricing section"). Use those insights to design the next test with a higher MDE or better hypotheses.
Do I need separate sample size calculations for each language?
Yes. Baseline conversion rates vary wildly by language. A language converting at 1% needs 4x the sample of one converting at 4% for the same MDE. Calculate per language, then take the maximum as your test duration driver.
Can I extend past 4 weeks if I'm close to sample size?
Only if seasonality is stable and you have a documented reason (e.g., "Black Friday traffic anomaly in week 3"). Otherwise, the 4-week cap protects you from novelty effects, cookie decay, and external validity threats. Design the next test with a larger MDE instead.
How does SeaText's Translation Agent fit into this workflow?
The Translation Agent deploys 125-language variants with zero code and full editorial control. You approve or edit machine translations before they go live. The CRO Testing Agent then runs continuous multi-armed bandit tests on those variants, using reading telemetry to find winners faster than binary conversion tracking allows.
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