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OCR H446 1.3.1 Compression need, lossy and lossless choice
Part 1 of 7 · H446 1.3.1 · Compression, encryption and hashing
H446 1.3.1 asks not which method makes the smaller file but what a use case needs once the data is reconstructed. This worksheet builds that reconstruction test and applies it to lossy and lossless choices across contexts where the right answer changes.
Students will:
- explain why storage, bandwidth and transfer time make compression worthwhile
- describe what happens to data across its original, compressed and reconstructed states
- identify a use that clearly requires exact reconstruction
- compare lossy and lossless compression using the same criterion on both sides
- recommend a method for a medical scan, speech over a slow link and a configuration file, with reasons
Inside: 5 explanation cells, 1 multiple-choice question, 2 fill-in-the-blanks cells and 2 written answers. 29 marks, about 35 to 45 minutes.
Series: H446 1.3.1 · Compression, encryption and hashing, part 1 of 7.
Shared by Coding PathwayVerified teacher
- 10 cells
- About 45 minutes
- CC BY-SA 4.0
- Shared 31 Aug 2026
- Updated 3 Sept 2026
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The whole resource, exactly as a class sees it. Answers and marking are held back.
Compression need, lossy and lossless choice
Compression represents data using fewer bits. The useful question is not simply “which method makes a smaller file?” It is “what consequence does this use case require when the data is reconstructed?”
By the end, you will be able to
- explain why storage, bandwidth and start/buffering time matter;
- distinguish exact reconstruction from an approximation;
- compare both sides and recommend from stated constraints.
Follow the data through three states
Lossless compression removes representational redundancy without losing the information needed to reconstruct the exact original. It is required when one altered character, value or instruction could change meaning.
Lossy compression permanently discards selected detail. It can often produce a smaller representation, but reconstruction is only an approximation. It can be acceptable when a human audience can tolerate the quality change. Neither label names one file type or guarantees a particular saving.
Worked comparison: live rehearsal stream
A theatre sends a rehearsal video to viewers whose connections sometimes deliver data more slowly than the original high-data-rate file requires.
- A smaller lossy version needs fewer bits to arrive for each part of the video. Playback may start sooner and pause to buffer less often on that constrained connection.
- A lossless version preserves every encoded detail but is likely to require more data for comparable source material.
- If the viewers only need to judge movement and timing, a controlled quality reduction may be acceptable, so lossy is the stronger recommendation.
“It downloads faster” is incomplete here: the demand is continuous playback, so connect fewer transferred bits to start time or buffering.
Which use most clearly requires lossless reconstruction?
- AA database export used to restore customer balances
- BA preview image used only as a small page thumbnail
- CA speech recording streamed over an unstable connection
- DA background photograph deliberately shown at reduced detail
- exact
- discards
- approximation
- fewer
- more
Compare lossy and lossless compression for the rehearsal stream and for a source-code archive. Use one matched criterion for both methods in each context, then recommend.
For the stream, connect size/data rate to start or buffering and acceptable quality. For source code, connect exact reconstruction to program meaning.
Students type their answer here.
Apply the model independently
The remaining tasks change the context or reduce the support. Complete them without copying the worked model, then check that each explanation connects a mechanism to its consequence.
Recommend lossy or lossless compression for each: (1) a medical scan retained for later numerical measurement, (2) spoken directions sent to a rescue team over a low-bandwidth link, (3) a configuration file that will be loaded by software, and (4) promotional music samples where a small quality reduction is acceptable. State the decisive consequence and one trade-off for each.
Use feature → context consequence → choice. Do not choose from a file-type label alone.
Students type their answer here.
Review your understanding
Before submitting, check that you can explain the main distinction in your own words, apply it in an unfamiliar context and justify each consequence rather than only naming a feature.