A/B Test Sample Size and the Peeking Problem: Interview Guide
Size an A/B test from baseline, MDE, alpha and power, turn it into days, and see by simulation why stopping at the first p below 0.05 fails.
~8 min readread →
psst… 2630+ questions inside
Developer interview questions & practice.
Prepare with explained interview questions and study notes for JavaScript, TypeScript, React, Angular, Java, Python, SQL, AWS and more, organized by topic and with the reasoning written in the margins. Finally, practice them like it's the real interview.
Closures, Ch. 1 ✦
function outer() { let count = 0; return function inner() { return ++count; }; } const next = outer(); next(); // 1 next(); // 2 ← why not 1?!
Q: var vs let?var → function scope, hoisted as undefined
let → block scope + TDZ!
let → block scope + TDZ!
Try it yourself ★Explain why the closure remembers count.
Pick a chapter (start anywhere, really)
See all chapters →Frontend
The browser, the language and the frameworks.JSCh. 01JavaScript
Closures, hoisting, prototypes & the event loop.
TSCh. 02TypeScript
Generics, narrowing, utility & mapped types.
ReactCh. 03React
Hooks, rendering, memoization & state.
AngularCh. 04Angular
Change detection, DI, signals & RxJS.
HTML/CSSCh. 05HTML & CSS
Semantics, the box model, layout & a11y.
NextCh. 24Next.js
App Router, Server Components, caching & rendering.
{ }BonusCoding round
Debounce, Promise.all, LRU cache… write it, run the tests.
Backend
Servers, services and the languages behind them.NodeCh. 06Node.js
Event loop phases, streams & modules.
JavaCh. 07Java
OOP, collections, JVM, concurrency & streams.
SpringCh. 08Spring Boot
IoC, auto-configuration, JPA, REST & security.
PythonCh. 09Python
Data model, generators, decorators & the GIL.
MicroCh. 10Microservices
Service design, messaging, resilience & sagas.
GoCh. 25Go
Goroutines, channels, interfaces & errors.
KafkaCh. 26Apache Kafka
Topics, partitions, consumer groups & delivery.
Databases
Relational and document stores, queries and design.Cloud & DevOps
Containers, orchestration, cloud and delivery.Platforms
Enterprise platforms with their own interview loops.Fundamentals
Algorithms and architecture: asked everywhere.DSACh. 18Data Structures & Algorithms
Big-O, arrays, trees, graphs & DP patterns.
DesignCh. 19System Design
Feeds, autocomplete, caching & performance.
NetCh. 20Networking
HTTP, DNS, TCP and request troubleshooting.
OSCh. 21Operating Systems
Processes, threads, memory and concurrency.
TestingCh. 22Software Testing
Test strategy, isolation, edge cases and debugging.
PeopleCh. 23Behavioral & HR
STAR stories, collaboration and project discussions.
AI & Machine Learning
From classic ML to LLM apps in production.MLCh. 28Machine Learning
Bias-variance, metrics, trees & ensembles.
DLCh. 29Deep Learning
Backprop, optimizers, CNNs & transformers.
AICh. 30AI & LLM Engineering
RAG, embeddings, agents, evals & guardrails.
MLOpsCh. 31MLOps
Pipelines, serving, monitoring & drift.
StatsCh. 32Statistics & Data Science
Probability, A/B tests & hypothesis testing.
The practice room no sign-up, saved in your browser
Open the practice room →Quiz1107 multiple-choice & “what's the output?” questions, with explanations.Flashcards1529 cards with spaced repetition, so they stick.Mock interviewTimed questions, model answers and a score report.Coding round14 challenges with tests that run in your browser.
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Latest notes fresh off the pen ✎
All 825 notes →Bayes' Theorem and the Base Rate Fallacy: Interview Guide
Solve the classic 1% disease test question three ways, see why rare events wreck precision, and avoid the base rate traps interviewers set.
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Central Limit Theorem Explained: Data Science Interview Guide
What the central limit theorem really promises, shown by simulation on skewed revenue data, and where it quietly breaks in A/B tests.
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Confidence Intervals Explained: Data Science Interview Guide
What 95% confidence really means, how to build t, Wilson and bootstrap intervals in Python, and why the textbook proportion interval fails.
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Linear Regression Assumptions: What Breaks and How to Check
The OLS assumptions interviewers expect, which violations bias coefficients and which only break standard errors, with Python checks for each.
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P-Values and Hypothesis Testing: Data Science Interview Guide
What a p-value actually measures, how to run a two-proportion test by hand in Python, and the misreadings that sink data science candidates.
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Tear-off cheat sheets print & stick on your wall
All cheat sheets →JavaScript Essentials
48 must-know facts & snippets
open & print →
TypeScriptFrom unknown to infer, on one page
open & print →
TypeScript Utility TypesPartial → Awaited, all of them
open & print →
ReactRendering to Server Components, on one page
open & print →
React HooksEvery hook, one example each
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AngularSignals to guards, standalone-first
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RxJS OperatorsThe 30 operators you'll actually use
open & print →
HTML & CSSSemantics to grid, cascade to stacking
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Next.jsApp Router, RSC, caching and actions
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Node.jsEvent loop to streams, checked on Node 22
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JavaOOP to virtual threads, on one page
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Spring BootBeans to JWT and @Transactional, one page
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PythonCore Python to asyncio, on one page
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MicroservicesDDD to sagas, retries and tracing
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SQL & PostgreSQLJoins to MVCC, every query run on Postgres 16
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MongoDBBSON to sharding, with the defaults that matter
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GoGoroutines to generics, slices to context
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DockerDockerfile to Compose, layers to PID 1
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KubernetesPods to Helm, plus a troubleshooting table
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RedisData types to Cluster slots, Redis 7.4/8
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AWSEC2 to IAM to DR, the services that come up
open & print →
Git & CI/CDStaging area to canary deploys, on one page
open & print →
Apache KafkaPartitions to transactions, KRaft era
open & print →
ServiceNowGlideRecord to update sets, on one page
open & print →
Data Structures & AlgorithmsBig-O to DP, with Python templates
open & print →
System DesignRendering to caching, plus five mini-designs
open & print →
Statistics & Data ScienceBayes to CUPED, p-values to regression
open & print →
Machine LearningBias-variance to boosting, metrics to leakage
open & print →
Deep LearningBackprop to KV cache, Adam to LoRA
open & print →
AI & LLM EngineeringTokens to agents, RAG to guardrails
open & print →
MLOpsPipelines to PSI, registry to rollback
open & print →
A note in your inbox, every Sunday.
One tricky interview question, fully explained. Takes 5 minutes to read with your coffee.