If you’ve ever shipped a feature and thought, “Did we actually make things better?”, you’re not alone. A/B testing is supposed to be our scientific answer to that question — but running good experiments takes more than sprinkling some feature flags and plotting a graph.

In practice, many teams learn experimentation the hard way. They launch tests with unclear hypotheses, biased assignments, or underpowered sample sizes, only to discover weeks later that their results are inconclusive or misleading. This means going back to the drawing board, restarting experiments, and losing valuable time — a hit to both product velocity and team morale.


Even worse, decisions made on noisy or misinterpreted data can lead teams to ship the wrong features, double down on bad ideas, or miss opportunities that would have moved the needle. The result is a slower feedback loop, wasted engineering cycles, and products that evolve by gut feel rather than evidence.


At scale, these problems compound. When you have millions of users, dozens of simultaneous tests, and machine learning models depending on clean signals, sloppy experimentation can quietly derail your roadmap. This is why A/B testing must be treated as an engineering discipline — one with rigor, guardrails, and repeatable processes that let teams move fast without breaking trust in their data.


This post lays out a set of battle-tested best practices for running experiments that not only produce reliable results, but actually help teams ship faster, learn more, and build better products.


1. Align on Goals and Hypotheses


Define the Purpose

Identify the user or business problem you want to solve (e.g., improving onboarding conversion).


Formulate a Hypothesis

Express it in a testable format:

“We believe that redesigning the onboarding screen will increase click through rate compared to the current experience.”

This keeps the team aligned on why the experiment exists.


2. Collaborate Early Across Teams

A/B tests succeed when product managers, engineers, data scientists, and designers work together:


3. Design the Experiment Carefully

Randomization & Segmentation

Use random assignment to avoid bias in the experiment


Sample Size & Duration

Calculate minimum sample size (power analysis) before launch to avoid underpowered tests.


Guardrails & Safety Checks

Define guardrail metrics (e.g., crash rate, latency, unsubscribe rate) to prevent harm.


4. Implement with Robust Engineering Practices


Here is a sampler Experiment Handler code that takes care of key aspects of user assignment to experiment branches, exposure logging and conversion logging. Exposure logging records when a user sees an impression of a variant, while conversion logging records when the user completes a desired action (click) after the exposure. 

class ABTestExperiment:
    """Sample A/B Test experiment handler with selected logging"""
    
    def __init__(self, config: ExperimentConfig):
        self.config = config
        self.logger = logging.getLogger(f'ab_test.{config.experiment_id}')
        
        # Log experiment initialization
        self.logger.info(f"Initialized experiment: {config.name}")
        self.logger.info(f"Traffic allocation: {config.traffic_allocation}")
    
    def assign_user_to_branch(self, user_id: str) -> ExperimentBranch:
        """Assign user to experiment branch using consistent hashing"""
        if not self.config.is_active:
            self.logger.warning(f"Experiment {self.config.experiment_id} is inactive")
            return ExperimentBranch.CONTROL
        
        # Create deterministic hash for consistent assignment
        hash_input = f"{self.config.experiment_id}:{user_id}"
        hash_value = hashlib.md5(hash_input.encode()).hexdigest()
        hash_number = int(hash_value[:8], 16) / (16**8)  # Convert to 0-1 range
        
        # Assign based on traffic allocation
        cumulative_allocation = 0.0
        assigned_branch = ExperimentBranch.CONTROL
        
        for branch, allocation in self.config.traffic_allocation.items():
            cumulative_allocation += allocation
            if hash_number <= cumulative_allocation:
                assigned_branch = branch
                break
        
        # Log assignment
        self.log_assignment(user_id, assigned_branch, hash_number)
        return assigned_branch
    
    def log_assignment(self, user_id: str, branch: ExperimentBranch, hash_value: float):
        """Log user assignment to experiment branch"""
        assignment_data = {
            'event_type': 'user_assignment',
            'experiment_id': self.config.experiment_id,
            'experiment_name': self.config.name,
            'user_id': user_id,
            'assigned_branch': branch.value,
            'hash_value': hash_value,
            'timestamp': datetime.utcnow().isoformat(),
        }
        
        self.logger.info(f"User assignment: {json.dumps(assignment_data)}")
    
    def log_exposure(self, user_id: str, branch: ExperimentBranch, context: Dict[str, Any] = None):
        """Log when user is exposed to experiment. This log is very critical to detect bias in experiments and make sure same user is not exposed to multiple variants"""
        exposure_data = {
            'event_type': 'experiment_exposure',
            'experiment_id': self.config.experiment_id,
            'experiment_name': self.config.name,
            'user_id': user_id,
            'branch': branch.value,
            'timestamp': datetime.utcnow().isoformat(),
            'context': context or {}
        }
        
        self.logger.info(f"Experiment exposure: {json.dumps(exposure_data)}")
    
    def log_conversion(self, user_id: str, branch: ExperimentBranch, 
                      conversion_type: str, value: float = None, 
                      metadata: Dict[str, Any] = None):
        """Log conversion event for analysis. This log tracks the key metrics used to determine experiment result. Eg : click through rate, click through sale, etc"""
        conversion_data = {
            'event_type': 'conversion',
            'experiment_id': self.config.experiment_id,
            'experiment_name': self.config.name,
            'user_id': user_id,
            'branch': branch.value,
            'conversion_type': conversion_type,
            'value': value,
            'timestamp': datetime.utcnow().isoformat(),
            'metadata': metadata or {}
        }
        
        self.logger.info(f"Conversion: {json.dumps(conversion_data)}")


5. Monitor in Real Time


6. Analyze with Statistical Rigor


7. Communicate and Document Results


8. Iterate and Build a Culture of Experimentation


In the era of data-driven product development and machine learning–powered features, experimentation isn’t just a tool — it’s the feedback loop that powers innovation. Teams that master it move faster, learn more, and build better products than those that rely on guesswork.

So the next time you spin up an experiment, ask yourself: are we treating this as a side project, or as the core engine that drives our product forward?