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spaced-repetition

spaced-repetition learning

Anki, Duolingo

SystemTechniques · 5Primitives · 22 spaced-repetition Incremental sync cursorsdistributed · medium: The client keeps one opaque token; each pull returns only the changes after it plus the next token, so sync cost scales with what changed, not with the dataset.›A/B testing (online controlled expe…statistics · medium: Randomly split units into arms by a salted hash, log exposure, compare metrics with a confidence interval, and guard the result with sample-ratio, guardrail and peeking checks.›Hypothesis testingstatistics · medium: Pick a test statistic, compare it to its distribution under "no effect", read off a p-value, and control error rates across one test or many.›Sequential hypothesis testingstatistics · medium: Group-sequential and alpha-spending designs for interim analyses without inflating errorSequential probability ratio teststatistics · medium: Add up the log-likelihood ratio per observation and stop when it crosses a fixed bound; the least data of any test with the same error rates (for two point hypotheses).›Write-ahead logstorage · low: Append each change to a checksummed sequential log and fsync it before touching data pages; the basis of crash durability in nearly every database.›d-ary heapdata-structures · low: Array heap with d children per node (d = 4 typical): half the binary heap's depth, siblings in one cache line, faster priority queues.›Linear and logistic regressionmachine-learning · low: Fit a weighted sum of features to a number (least squares) or a probability (logistic); the baseline every model must beat.›Spaced repetitionmachine-learning · low: Review scheduling by memory models (SM-2, FSRS) predicting recall probabilityL-BFGSnumerics · low: Quasi-Newton minimizer that approximates the inverse Hessian from the last m gradient differences via the two-loop recursion; O(m·n) memory and work per step.›Item response theorystatistics · low: Answer probability is a logistic curve of ability minus item difficulty (1PL), with item slope (2PL) and a guessing floor (3PL); fit items by marginal ML, score people, measure precision by information.›B+ treestorage · low: Balanced high-fanout search tree with all values in linked leaves; the default on-disk index for point lookups and range scans.›Calendar date arithmeticsystems · low: Civil dates, time zones (tzdb), DST gaps, month-end anchoring and duration mathMessage sequence numbersdistributed · low: A per-stream counter on every message lets the receiver detect loss, duplicates and reordering, ack cheaply and resume after reconnect with "send from N".›Cursor paginationnetworking · low: Keyset/opaque-cursor pagination for stable incremental reads from APIs and tablesBootstrap confidence intervalsstatistics · low: Resampling-based confidence intervals for arbitrary statisticsCUPED (variance reduction with pre-…statistics · low: Subtract the part of each unit's outcome that a pre-experiment covariate predicts; the effect estimate stays unbiased and its variance shrinks by 1 - rho^2.›Experiment layering (overlapping ex…statistics · low: Partition the tunable parameters into layers, hash each unit with a per-layer salt so layers are independent, and give experiments inside a layer disjoint bucket ranges.›Deterministic bucketingsystems · low: Hash a stable unit id with a per-purpose salt into a bucket; "in" means bucket below the percentage, so assignment is stateless, repeatable and only grows on ramp-up.›Wilson score intervalstatistics · low: Wilson lower bound for ranking by up/down votes and proportions with few samplesBayesian rate estimationstatistics · low: Estimate a conversion or event rate from few observations with a Beta-Binomial or Gamma-Poisson update: two counters, an honest interval, and P(B beats A) directly.›+ 6 more primitives