When to Use A/B Testing
A/B testing is powerful but not appropriate for every situation:Ideal Use Cases
Ideal Use Cases
Consumer applications with high volume:
- Applications with thousands of daily users (sufficient sample size)
- Use cases where small quality variations are acceptable
- Scenarios where you can collect quality signals (user feedback, automated scores)
- Youâve validated improvements on test datasets
- You want to verify production performance before full rollout
- You can monitor metrics in real-time to catch issues early
- Incremental prompt improvements where directional changes are clear
- Testing hypotheses about what drives quality (tone, length, structure)
- Comparing prompts with similar expected performance
Avoid A/B Testing For
Avoid A/B Testing For
Mission-critical applications:
- Healthcare decisions (potential patient harm)
- Financial transactions (regulatory requirements)
- Legal advice (liability concerns)
- Safety-critical systems (autonomous vehicles, industrial controls)
- Fewer than 100 daily users (insufficient statistical power)
- Use cases with long feedback cycles (weeks between samples)
- Scenarios where each request is unique (no aggregate patterns)
- Applications where any error is unacceptable
- Regulated industries with strict compliance requirements
- Use cases requiring deterministic outputs
Prerequisites for Successful A/B Testing
Prerequisites for Successful A/B Testing
Before starting A/B testing, ensure you have:
- Measurable success metrics: Quality scores, user feedback, task completion rates, or business outcomes
- Sufficient traffic volume: At least 100-200 samples per variant for statistical significance
- Prompt linking infrastructure: Ability to link prompts to traces for metric aggregation
- Monitoring dashboards: Real-time visibility into quality metrics by prompt version
- Rollback capability: Ability to stop the test and revert if issues arise
- Statistical analysis skills: Understanding of significance testing, confidence intervals, and statistical power
How A/B Testing Works
Complete workflow from setup to decision: The A/B testing lifecycle: Create variants â Implement random assignment â Collect sufficient data â Analyze for statistical significance â Make deployment decision â Monitor results.Create prompt variants and assign labels
Create two (or more) prompt versions with different content, structure, or parameters:Via ABV UI:Version numbers: ABV automatically assigns incremental version numbers (e.g., versions 3 and 4), but youâll reference by label in your code.
- Navigate to your prompt in the ABV dashboard
- Create a new version with variant A content
- Assign label
variant-a(orprod-a) - Create another version with variant B content
- Assign label
variant-b(orprod-b)
Implement randomized assignment in application code
Modify your application to randomly select between variants for each request:Python implementation:TypeScript/JavaScript implementation:Traffic split ratios: Use 50/50 for equal comparison, or adjust ratios (e.g., 90/10 for cautious canary deployment).
Collect data over sufficient time period
Run the A/B test until youâve collected enough data for statistical significance:Minimum sample size:
- At least 100-200 generations per variant
- More samples for smaller expected differences
- Use online sample size calculators for precise requirements
- Run for multiple days to account for day-of-week effects
- Include weekdays and weekends if usage patterns differ
- Ensure you capture diverse user segments and use cases
- Watch dashboards for unexpected issues
- Check that traffic is splitting as expected
- Verify metrics are being collected for both variants
- One variant shows severe quality degradation
- Error rates spike for one variant
- Statistical significance is achieved with clear winner
Analyze results and calculate significance
Navigate to the prompt in ABV dashboard and compare metrics by version:Key metrics to compare:Tools for analysis: Use Python (scipy, statsmodels), R, or online calculators for significance testing.
- Quality scores: Median score, score distribution by variant
- Latency: Median, p95, p99 response times
- Token usage: Input tokens, output tokens (affects cost)
- Cost: Median cost per generation
- User feedback: Thumbs up/down ratios, satisfaction ratings
- Use significance tests (t-test, Mann-Whitney U test) to determine if differences are real
- Calculate confidence intervals (95% CI recommended)
- Consider practical significance: Is the improvement meaningful even if statistically significant?
Make decision and deploy winner
Based on analysis, choose the winning variant:Clear winner:Post-deployment monitoring: Continue monitoring quality after full rollout to ensure results hold at 100% traffic.
- Variant significantly better on primary metric (quality)
- No significant degradation on secondary metrics (cost, latency)
- Action: Promote winner to production by reassigning
productionlabel
- Variant better on quality but worse on cost
- Small improvement with high uncertainty
- Action: Evaluate tradeoffs, possibly run longer test, or choose based on business priorities
- Variants perform similarly across all metrics
- Action: Keep existing version (simpler) or choose based on maintenance/cost
Implementation Examples
Complete examples for both SDKs:Python SDK Implementation
Python SDK Implementation
Complete A/B testing implementation:Weighted traffic split (90% control, 10% variant):
JavaScript/TypeScript SDK Implementation
JavaScript/TypeScript SDK Implementation
Complete A/B testing implementation:Setup (A/B test implementation (Weighted traffic split (90% control, 10% variant):
instrumentation.ts):index.ts):Statistical Analysis
Understanding statistical concepts for A/B testing:Statistical Significance (p-value)
Statistical Significance (p-value)
Definition: Probability that observed difference occurred by random chance.Interpretation:
- p < 0.05: Less than 5% chance results are due to randomness (commonly used threshold)
- p < 0.01: Less than 1% chance (stronger evidence)
- p > 0.05: Difference not statistically significant (could be random)
- Variant A: median score 4.2
- Variant B: median score 4.5
- p-value: 0.03
- Conclusion: The 0.3 point improvement is statistically significant (p < 0.05)
Confidence Intervals
Confidence Intervals
Definition: Range where the true value likely falls.Interpretation:
- 95% CI: Weâre 95% confident the true value is in this range
- Wider intervals indicate more uncertainty
- Non-overlapping intervals suggest significant difference
- Variant A: median score 4.2, 95% CI [4.0, 4.4]
- Variant B: median score 4.5, 95% CI [4.3, 4.7]
- Conclusion: Intervals donât overlapâvariant B is likely better
Sample Size and Statistical Power
Sample Size and Statistical Power
Statistical power: Probability of detecting a real difference if it exists.Factors affecting required sample size:
- Effect size: Smaller differences need more samples
- Baseline variance: Higher variance needs more samples
- Desired power: Higher power (80-90% recommended) needs more samples
- Significance level: Stricter thresholds (p < 0.01) need more samples
- Baseline score: 4.0 (std dev 1.0)
- Expected improvement: 10% (0.4 points)
- Desired power: 80%
- Significance: 0.05
- Required samples: ~400 per variant
Common Statistical Tests
Common Statistical Tests
For continuous metrics (quality scores, latency):
- t-test: Compares means, assumes normal distribution
- Mann-Whitney U test: Compares medians, no distribution assumption (recommended for scores)
- Chi-square test: Compares proportions
- Fisherâs exact test: For small sample sizes
- Poisson test: Compares event rates
Common Pitfalls to Avoid
Stopping Tests Too Early
Stopping Tests Too Early
Problem: Declaring a winner after 50 samples because variant B looks better.Why itâs wrong: Small samples have high variance. Early results often donât hold with more data.Solution: Pre-commit to minimum sample size (100-200+ per variant) before looking at results. Use sequential testing methods if you must peek early.
P-Hacking (Data Dredging)
P-Hacking (Data Dredging)
Problem: Running multiple tests on the same data until you find statistical significance.Example: Testing 20 different metrics, finding that 1 is significant at p < 0.05 (expected by chance).Solution: Pre-register your primary metric before starting the test. Treat secondary metrics as exploratory only.
Ignoring Practical Significance
Ignoring Practical Significance
Problem: Deploying a variant because itâs statistically better, even though the improvement is tiny.Example: p < 0.01 but quality improves only 0.5% while cost increases 30%.Solution: Set minimum thresholds for practical significance before the test. Consider cost-benefit tradeoffs.
Not Linking Prompts to Traces
Not Linking Prompts to Traces
Problem: Implementing A/B test but forgetting to link prompts to generation spans.Result: ABV canât aggregate metrics by prompt version. You have no way to compare variants.Solution: Always pass
prompt=selected_prompt when creating generation spans:Confounding Variables
Confounding Variables
Problem: Running variant A during weekdays and variant B during weekends, then concluding B is better.Why itâs wrong: Weekend traffic might differ from weekday traffic. You canât tell if the difference is due to the prompt or the day of week.Solution: Run variants concurrently with randomized assignment to ensure comparable populations.
Next Steps
Link Prompts to Traces
Essential setup for tracking metrics by prompt version
Version Control
Manage prompt versions and labels for A/B testing
Get Started with Prompts
Create and fetch prompts with the ABV SDK
Prompt Experiments
Offline evaluation as a complement to A/B testing
Scores Data Model
Understand quality scores used in A/B test analysis
Metrics Dashboard
Analyze and visualize A/B test results