Meaning
Statistical hypothesis testing procedure allows sample sizes to remain undetermined until accumulated data crosses predefined decision boundaries. The sequential probability ratio test evaluates likelihood ratios continuously after each individual observation arrives. Founders apply this sequential probability ratio test to reduce observation costs during operational experiments where data collection consumes scarce attention or capacity.
Traditional fixed sample methods fix sample sizes beforehand, whereas this methodology stops sampling the moment mathematical thresholds demonstrate sufficient statistical certainty. The procedure calculates two distinct bounds corresponding to accept and reject decisions based on predetermined error rates.
Decision Boundary
Two parallel lines defined by upper and lower limits govern stopping rules in the evaluation process. The likelihood ratio accumulates with every incoming data point and drifts toward either boundary until a threshold is crossed. Mathematical derivation establishes these boundaries directly from chosen Type I and Type II error probabilities without requiring variance estimations.
Crossing the upper limit terminates the trial with acceptance of the alternative hypothesis, while crossing the lower limit terminates the trial with acceptance of the null hypothesis. Falling between both limits forces the collection of another observation without rendering a verdict.
Sample Economy
Stopping rules reduce average sample sizes significantly compared to standard fixed designs under identical statistical power requirements. Observations cease early when signal strength is high, which spares observation capacity during periods of scarce attention. Average sample number functions mathematically as an expected value that varies according to true parameter states.
High variance conditions reduce this efficiency advantage because data trajectories oscillate longer between bounds before crossing either threshold.
Terminal State
Final disposition of the test yields a discrete binary outcome accompanied by a measured likelihood ratio. This calculation avoids arbitrary stopping points driven by impatience or resource exhaustion during high cost operational phases. Mathematical guarantees bound error rates tightly regardless of when the stopping boundary is crossed.
Subsequent operational decisions rely on this terminal classification without needing further confirmation from larger datasets.