Explaining what is lost and what is kept when a file is compressed

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Date:

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Quick Questions

Shade in the A or B circle for each question.

1. Which sentence is a clearer explanation of JPEG compression?

A JPEG makes a photo smaller because it is a good file type.

B JPEG makes a photo smaller because it throws away tiny colour details your eyes barely notice.

2. Which explanation of run-length encoding (RLE) states what is kept?

A RLE makes files smaller because it is a clever trick.

B RLE stores 'W5' instead of 'WWWWW' because repeats can be counted, and no data is lost.

3. Mia says: 'MP3 is smaller because it drops sounds most people can't hear.' What does her sentence do well?

A It names the technique, says how the size shrinks, and says what is lost.

B It only names the file type and says nothing about what is lost.

4. Which sentence uses 'because' to link a cause to a result correctly?

A A ZIP file can be restored exactly because lossless compression keeps all the original data.

B A ZIP file can be restored exactly because it has a smaller icon.

5. You are explaining lossy compression to a younger student. Which is the better explanation?

A It is like shrinking a drawing by leaving out tiny details, so the file is smaller but you can't get the details back.

B It applies a perceptual quantisation algorithm to the discrete cosine transform coefficients.

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1RLE shrinks data because it stores a count and a value instead of repeating the value.		AIt never says how the file shrinks or what is lost, and it has no because. A clear explanation links the method to the result and states what is lost or kept.
2JPEG is smaller because it throws away fine detail your eyes barely notice.		BA clear explanation of RLE: it names the technique, says how it shrinks the file, and states that nothing is lost.
3Explaining to a younger student what is kept by a lossless method, using because		CA ZIP file can be unzipped exactly because no data is thrown away, it is only packed more tightly.
4A sentence starter that links a lossy method to what is lost		DMP3 files are small because they remove sounds most people cannot hear, so some audio is lost for good.
5JPEG makes files smaller. It is good for photos.		EThe sentence wrongly says that data is lost. RLE is lossless, so the original can be rebuilt exactly from the counts and values.
6RLE is lossy because it replaces repeats with counts, so the original cannot be restored.		FA clear explanation of JPEG: it names the technique, says why the file is smaller, and states that detail is lost.

Definitions

because

Fill in the Blanks

Complete each line of code by filling in the missing parts.

RLE is lossless _______ no data is lost.

___ is smaller because it drops sounds you cannot hear.

RLE is ________ because nothing is lost.

JPEG is _____ because it removes detail.

A ZIP file is lossless because every bit of the original is ____.

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A MP3 is smaller than WAV

B because it adds extra sounds.

C Those sounds are lost forever.

Question 2

A RLE is lossy compression

B It replaces runs with counts

C so no pixels are lost.

Question 3

A A ZIP file is lossless

B because some data is deleted.

C You get the exact original.

Question 4

A Explaining JPEG to a Year 5:

B A JPEG is a lossy photo file.

C It merges similar colours

D so the file is much bigger.

E Fine detail is lost forever.

Question 5

A RLE deletes every repeated run

B and a value, so AAAA is 4A.

C It works because data repeats.

Answer Key

Explaining what is lost and what is kept when a file is compressed

Section A: Quick Questions

B — JPEG makes a photo smaller because it throws away tiny colour details your eyes barely notice.

B — RLE stores 'W5' instead of 'WWWWW' because repeats can be counted, and no data is lost.

A — It names the technique, says how the size shrinks, and says what is lost.

A — A ZIP file can be restored exactly because lossless compression keeps all the original data.

A — It is like shrinking a drawing by leaving out tiny details, so the file is smaller but you can't get the details back.

Section B: Match the Pairs

RLE shrinks data because it stores a count and a value instead of repeating the value. → A clear explanation of RLE: it names the technique, says how it shrinks the file, and states that nothing is lost.

JPEG is smaller because it throws away fine detail your eyes barely notice. → A clear explanation of JPEG: it names the technique, says why the file is smaller, and states that detail is lost.

Explaining to a younger student what is kept by a lossless method, using because → A ZIP file can be unzipped exactly because no data is thrown away, it is only packed more tightly.

A sentence starter that links a lossy method to what is lost → MP3 files are small because they remove sounds most people cannot hear, so some audio is lost for good.

JPEG makes files smaller. It is good for photos. → It never says how the file shrinks or what is lost, and it has no because. A clear explanation links the method to the result and states what is lost or kept.

RLE is lossy because it replaces repeats with counts, so the original cannot be restored. → The sentence wrongly says that data is lost. RLE is lossless, so the original can be rebuilt exactly from the counts and values.

Section C: Definitions

because — Linking word in an explanation: name the technique, say how it shrinks the file, and state what is lost or kept, e.g. MP3 is smaller because it drops sounds you can't hear.

Section D: Fill in the Blanks

RLE is lossless because no data is lost.

MP3 is smaller because it drops sounds
you cannot hear.

RLE is lossless because nothing is lost.

JPEG is lossy because it removes detail.

A ZIP file is lossless because every bit
of the original is kept.

Section E: Spot the Error

B — MP3 gets smaller by dropping sounds, not adding them. The line should say 'because it drops quiet sounds.'

A — RLE is lossless because every pixel can be rebuilt from the counts. The line should say 'RLE is lossless compression', which also matches line 3.

B — Lossless means no data is deleted, so the explanation must say 'because nothing is removed.' Deleting data would be lossy.

D — Merging similar colours reduces the data, so the file becomes smaller, not bigger. The line should say 'so the file is smaller.'

A — RLE does not delete runs. It stores each run as a count and a value, which is what line 2 describes. The line should say 'RLE stores a run as a count'.