AP Computer Science Principles · Unit 2 · Data
Use this AP CSP Unit 2 cheat sheet when you need a fast final review before a quiz, practice test, or full Unit 2 practice set. It summarizes the highest-yield Data unit ideas: binary numbers, bits and bytes, binary-to-decimal conversion, compression ratios, lossless vs lossy compression, metadata, big data, privacy, PII, and data bias.
This page is not meant to replace the full Unit 2 Review or Notes pages. Use it after you have learned the concepts, then take the quiz or full practice set to check whether you can apply them.
This AP CSP Unit 2 cheat sheet covers the fast facts students need before practice: binary place values, bit and byte conversions, binary-to-decimal steps, compression ratios, lossless vs lossy compression, metadata clues, big data benefits, privacy risks, PII, re-identification, and data bias.
In one sentence: AP CSP Unit 2 is about how computers represent, reduce, organize, analyze, and responsibly use data.
Use this cheat sheet as a recall tool, not a first lesson. First, scan each cheat sheet block. Second, cover the page and try to explain the rule from memory. Third, use the 5-minute checklist to decide whether you are ready for the Unit 2 quiz or full practice questions.
If a row feels confusing, do not stay on this page. Open the linked concept guide, review that topic, then return to the cheat sheet.
| Minute | What to Review | Goal |
|---|---|---|
| 0–1 | Binary, bits, and bytes | Remember place values and 8 bits = 1 byte |
| 1–2 | Binary to decimal | Add only the place values under 1s |
| 2–3 | Compression | Separate ratio, file size, and quality |
| 3–4 | Metadata and privacy | Identify what data reveals |
| 4–5 | Big data and bias | Name one benefit and one risk |
After the 5-minute review, close the cheat sheet and take the Unit 2 quiz without looking. Your missed questions show which concept page to reopen.
| Concept | Cheat Sheet Rule |
|---|---|
| Bit | One binary digit: 0 or 1 |
| Byte | 8 bits |
| Binary | Base-2 number system |
| Decimal | Base-10 number system |
| Digital data | Text, images, audio, video, and files can be represented with bits |
If the question says “bit,” think one 0 or 1. If it says “byte,” remember 8 bits.
Mbps and MB are not the same. Lowercase b usually means bits. Uppercase B usually means bytes.
Start from the right. Write place values as powers of 2: 1, 2, 4, 8, 16, 32, 64, 128. Add only the place values where the binary digit is 1.
| Binary | Work | Decimal |
|---|---|---|
| 101 | 4 + 1 | 5 |
| 1010 | 8 + 2 | 10 |
| 1011 | 8 + 2 + 1 | 11 |
| 1100 | 8 + 4 | 12 |
| 1111 | 8 + 4 + 2 + 1 | 15 |
| 10000 | 16 | 16 |
A binary number ending in 1 is odd. A binary number ending in 0 is even.
Binary 101 is not one hundred one. It equals decimal 5.
| Concept | Cheat Sheet Rule |
|---|---|
| Compression | Reduces file size |
| Main benefit | Less storage and faster transfer |
| Compression ratio | Original size ÷ compressed size |
| Percent saved | Amount reduced ÷ original size × 100 |
| Main tradeoff | Smaller file size may reduce quality or detail |
| Original | Compressed | Ratio | Percent Saved |
|---|---|---|---|
| 10 MB | 5 MB | 2:1 | 50% |
| 12 MB | 3 MB | 4:1 | 75% |
| 100 MB | 20 MB | 5:1 | 80% |
If the question asks for ratio, answer original : compressed. If it asks for percent saved, calculate the reduction compared with the original.
Do not answer 75% when the question asks for 4:1. Ratio and percent saved are different.
| Feature | Lossless | Lossy |
|---|---|---|
| Data removed? | No | Yes |
| Exact reconstruction? | Yes | No |
| Best for | Text, code, legal files, medical files | Photos, audio, video, streaming |
| Main benefit | Preserves original | Smaller files |
| Main tradeoff | May not shrink as much | May lose quality/detail |
Does the file need to be restored exactly? Yes → lossless. No, and smaller size matters → lossy may be better.
Lossy compression is not automatically bad. It is often the best choice for photos, music, and video when smaller files matter.
| Concept | Cheat Sheet Rule |
|---|---|
| Metadata | Data about data |
| File metadata | File size, type, author, creation date |
| Photo metadata | GPS location, timestamp, camera model |
| Email metadata | Sender, receiver, subject, timestamp |
| Privacy risk | Metadata can reveal hidden details |
Ask: Is this the content itself, or a detail describing the content? If it describes the content, it is metadata.
Compression does not automatically remove metadata. Removing EXIF or location metadata is a separate privacy step.
| Concept | Cheat Sheet Rule |
|---|---|
| Big data | Large, complex, fast, or varied datasets |
| PII | Data that can identify a person |
| Re-identification | Anonymous-looking data linked back to a person |
| Data bias | Incomplete or unrepresentative data creates unfair outcomes |
| Strong AP answer | Benefit + risk + specific example |
PII examples: Name, email, phone number, address, student ID, location data, biometric data, health data, login information.
For big data questions, always give both sides: one useful benefit and one specific risk.
Do not say big data is only good or only bad. AP CSP expects benefits and risks.
| Trap | What to Remember |
|---|---|
| Reading binary like decimal | 1012 = 510, not 101 |
| Mixing bits and bytes | 8 bits = 1 byte |
| Confusing Mbps and MB | Mbps is speed; MB is file size |
| Inverting compression ratio | Use original ÷ compressed |
| Treating lossy as bad | Lossy is useful when smaller files matter |
| Choosing lossy for exact files | Legal, code, spreadsheet, and medical files need lossless |
| Ignoring metadata privacy | GPS, timestamp, author, and device can reveal private details |
| Calling correlation causation | Related variables do not prove cause |
| Saying big data is only good | Big data has benefits and risks |
| Ignoring biased data | Bad training data can create unfair outcomes |
| Need | Shortcut |
|---|---|
| Convert bits to bytes | bits ÷ 8 |
| Convert bytes to bits | bytes × 8 |
| Count values from n bits | 2ⁿ possible values |
| Max unsigned value from n bits | 2ⁿ − 1 |
| Binary place values | 1, 2, 4, 8, 16, 32, 64, 128 |
| Compression ratio | original size ÷ compressed size |
| Percent saved | amount reduced ÷ original size × 100 |
| Lossless clue | exact reconstruction |
| Lossy clue | smaller file with acceptable quality loss |
| Metadata clue | data about data |
Before you take the AP CSP Unit 2 quiz or practice questions, make sure you can:
If you can do these without checking notes, take the Unit 2 quiz. If several items feel weak, open the linked concept guide before full practice.
| If you need… | Go here |
|---|---|
| Full explanation | Unit 2 Review |
| Concise explanation | Unit 2 Notes |
| Vocabulary recall | Unit 2 Flashcards |
| Short readiness check | Unit 2 Quiz |
| Full practice | Unit 2 Practice Questions |
| Binary help | Binary to Decimal Conversion |
| Compression help | Lossless vs Lossy Compression |
| Privacy/bias help | Big Data, Privacy, and Data Bias |
No. A cheat sheet is for fast recall after you have already learned the unit. Use it five minutes before the Unit 2 quiz or practice set, then answer without looking. If you cannot explain a row from memory, open the linked concept guide instead of rereading the table.
Binary place values, bit and byte rules, binary-to-decimal steps, compression ratio and percent saved, lossless versus lossy picks, metadata clues, PII, re-identification, data bias, and common traps. This page groups those into six cheat sheet blocks plus a five-minute review path.
Bits to bytes (divide by 8), bytes to bits (multiply by 8), 2ⁿ values from n bits, max unsigned 2ⁿ − 1, compression ratio as original divided by compressed size, and percent saved as amount reduced divided by original size times 100.
Ask whether the file must be restored exactly. Yes means lossless (text, code, legal, medical). No, and smaller size matters, often means lossy (photos, audio, video). Lossy is not automatically bad when quality loss is acceptable.
Reading binary like base-10. Binary 101 equals decimal 5, not one hundred one. Also mix up bits and bytes, or Mbps (speed in bits) with MB (file size in bytes).
Inverting the ratio or answering with percent saved when the stem asks for ratio. Use original divided by compressed for ratio (for example 12 MB to 3 MB is 4:1). Percent saved is a different calculation.
Yes. Cheat Sheet 5 covers metadata (data about data) and privacy risks such as GPS in photos. Cheat Sheet 6 covers PII, re-identification, data bias, and big-data benefits and risks. Open the metadata and big-data guides for scenario practice.
Stop when you can complete the readiness checklist without notes and score well on the Unit 2 quiz and practice questions without opening the page. If you need the sheet on every question, you are not exam-ready yet.
Take the Unit 2 quiz without looking, then the fifty practice MCQs. Reopen concept guides for any missed topic. Use Unit 2 Review for full explanations, Notes for a concise outline, and Flashcards for vocabulary you keep missing.
No. This page does not provide official AP CSP test answers or secure classroom assessment answers. It gives original study guidance, formulas, traps, and links to AP-style practice so students can prepare honestly.