Worked example: File Formats and Quality

What this is

A secure Year 8 sheet with the correct Part A table, so you can mark the class at the front of the room. Part A is right or wrong; Part B is right only if the reason names a size or a quality note from the stimulus sheet. Show this after Part B is finished.

Part A — the format fact table

Format	Lossy or lossless?	Bitmap or vector?	See-through areas?	Animation?	The job it is best at
JPEG	Lossy. Detail is thrown away and cannot be got back.	Bitmap	No — see-through areas come out solid white.	No	Photographs for websites and email. 212 KB instead of 1.87 MB for the same photo.
PNG	Lossless. Every pixel is kept.	Bitmap	Yes, with soft edges.	No — the badge came out frozen on one frame.	Logos on top of photographs, screenshots, and master copies you will edit again.
GIF	Lossless, but only 256 colours per frame, so the photo's sky went stripy.	Bitmap	Yes, but only fully on or off, so edges look jagged.	Yes	Short, simple animations that must play anywhere.
SVG	Lossless — it is text describing shapes, so there is nothing to lose.	Vector. No pixels at all.	Yes	Yes	Logos, icons and diagrams at any size, from a badge to an A1 banner. 3 KB.
WebP	Either. It can be saved lossy (like JPEG) or lossless (like PNG).	Bitmap	Yes	Yes	Website images where you want JPEG quality at a smaller size — 138 KB against 212 KB.

Evidences criteria 1, 2 and 3: WebP is correctly given as "either", SVG is the only vector row, and GIF's two limits (256 colours, hard-edged transparency) are separated from PNG's.

The same photograph, five ways — drawn to scale

The WebP bar is one twentieth of the BMP bar, and on screen the two pictures look the same. That gap is what compression buys you.

Criterion 4: the chart is drawn from the stimulus sheet's real numbers, to scale, so the choice in Part B can be justified with a size rather than a feeling.

Part B — the five jobs

The job	Format	Reason
1. Litter-pick photograph on the website home page	JPEG	It is a photograph, so lossy is fine: 212 KB instead of 1.87 MB, and you cannot see the loss without zooming to 400%. A page full of PNGs would crawl on a phone. WebP would be better still at 138 KB if the website accepts it.
2. Logo on an A1 banner	SVG	Vector, so it is redrawn at whatever size the printer needs and stays razor sharp. The 1200 × 800 pixel versions go blurry blown up to A1. It is also the smallest file on the sheet at 3 KB.
3. Logo on top of a dark green photograph	PNG	It needs a see-through background with soft edges. JPEG cannot do transparency at all — the stimulus sheet says the background turned solid white and grey speckles appeared round the letters, and it was 94 KB against PNG's 19 KB, so it was bigger and worse. GIF would work but its edges are jagged.
4. Spinning badge in the newsletter email	GIF	It has to animate, which rules out PNG. The WebP is much smaller, 96 KB against 480 KB, but some email programmes still show it as a single frozen frame, and an email has to work for everybody. For the website version I would use the WebP.
5. Master copy for the shared drive, to be re-edited	PNG	Lossless, so next term's edit starts from every original pixel. A JPEG loses a little more every time it is re-saved, so after four rounds of editing the sky would be mush. 1.87 MB on a shared drive is a fair price for that.

Criteria 4 and 5: every row names a number or a named limitation, and rows 4 and 5 give an honest "it depends where it is going" answer rather than a single favourite format.

Part C — the sums

C1. 25 MB ÷ 1.87 MB = 13.36, so 13 PNGs. Check: 13 × 1.87 = 24.31 MB, which fits; 14 × 1.87 = 26.18 MB, which does not.

C2. 25 MB = 25 × 1024 = 25,600 KB. 25,600 ÷ 212 = 120.75, so 120 JPEGs. Check: 120 × 212 = 25,440 KB = 24.84 MB, which fits.

C3. The thrown-away detail matters for the master copy in the shared drive, because every re-save loses more and the damage builds up, and it matters if the photo is going to be printed large. It does not matter for a thumbnail on the website home page, where the picture is shown 300 pixels wide and nobody will ever see the difference.

Criterion 1 and 5: the arithmetic is shown and checked both ways, and the answer to C3 links lossy compression to repeated re-saving rather than treating "lossy" as simply bad.

Part D — the real exports

What I saved	Size	At 400% zoom
PNG	1.9 MB	Clean. Every branch on the tree is separate.
JPEG, high quality	260 KB	Almost clean. A faint haze where the hill meets the sky.
JPEG, low quality	38 KB	Obvious little squares, about eight pixels across, all over the sky, and the branches have smeared together.

I would hand in the high-quality JPEG for the website, because it is 260 KB and the haze is invisible at normal size — but I would keep the PNG in my folder as the master, because that is the one I can edit again next term without making it any worse.

Criterion 5: two different files for two different purposes, with the master copy kept lossless.

Why this response is secure

The fact table is correct on the points pupils usually get wrong: WebP can be lossy or lossless, GIF is limited to 256 colours per frame, GIF transparency is on-or-off, and SVG is the only vector format here.
Every format choice is justified with a real number from the stimulus sheet, and the chart is drawn to scale from those numbers.
It notices that JPEG was both bigger and worse for the flat logo — 94 KB against 19 KB — which shows the pupil is reasoning about the content, not ranking formats by size.
The arithmetic in Part C is shown and checked, and the answers are whole files rather than 13.36 of one.
It separates the file you publish from the file you keep, which is the storing-and-sharing point the lesson is mapped to.