Worked example · Design Technology · Year 8
DCF: Citizenship → Health and well-being
Also assessed, LNF: Numeracy → Statistics and probability → Interpreting data
A secure Year 8 response, annotated against the success criteria. Key answers: desk 550 to 700 mm = 55 to 70 cm (15 cm of adjustment); top of screen 1020 to 1240 mm = 102 to 124 cm above the floor (22 cm); mouse reach set by the 5th percentile, 38 cm. Log: mean 215 min (3 h 35 min), range 270 min, median 195 min; Bethan, Deio and Iestyn broke the rule, 30%. Gethin, at exactly 50 minutes, kept it. The numeracy evidence for the LNF judgement is Step 2 and Step 3: look for working shown, correct units and a corrected conclusion.
(a) 95% of pupils aged 12 to 14 have a sitting eye height below 790 mm, and only 5% are taller. So if the screen can go high enough for a 790 mm eye height, it suits everyone except the tallest 5%.
(b) 41 cm is the middle value, so about half the pupils have a knee-to-floor height below it and their feet would not reach the floor, and about half are above it, with their knees pushed up. A fixed chair fits the “average” pupil, but most pupils are not exactly average.
(c) 5% of pupils are below the 5th percentile and 5% are above the 95th, so 100 − 5 − 5 = 90% are in between. The smallest 5% and the largest 5% are left out. They can still be helped with a footrest, a cushion, or a bigger or smaller chair kept in the room.
Explains a percentile as “what percentage are below this value”, shows why the average fails and where 90% comes from, and offers fixes for the 10% left out. Evidences criterion 1.
| Setting | 5th percentile (mm) | 95th percentile (mm) | Adjustment (mm) | Range in cm |
|---|---|---|---|---|
| Chair seat | 370 | 450 | 450 − 370 = 80 | 37 to 45 cm (8 cm) |
| Desk | 370 + 180 = 550 | 450 + 250 = 700 | 700 − 550 = 150 | 55 to 70 cm (15 cm) |
| Top of screen | 370 + 650 = 1020 | 450 + 790 = 1240 | 1240 − 1020 = 220 | 102 to 124 cm (22 cm) |
(b) The 5th percentile, 380 mm = 38 cm. Reach is set by the smallest user: if a pupil with the shortest forearm can reach the mouse, everyone with a longer forearm can too. Using the 95th (46.5 cm) would make 95% of pupils stretch.
(c) The chair goes up to 50 cm, which covers the 95th percentile (45 cm) with room to spare. But its lowest setting is 40 cm, which is 40 − 37 = 3 cm too high for the 5th percentile pupil. Every pupil with a knee-to-floor height under 40 cm would have their feet off the floor. From the table that is more than 5% but fewer than 50% of pupils, because 40 cm is between the 5th (37 cm) and the 50th (41 cm). A footrest up to 3 cm high solves it.
Every value is read from the right row and column, the working is shown in mm, and each answer is converted to cm correctly. The chair check uses the table to estimate how many pupils are affected rather than guessing. Evidences criterion 2 and is the first piece of numeracy evidence.
| Question | Working and answer |
|---|---|
| Mean total screen time | 180 + 240 + 150 + 420 + 200 + 160 + 210 + 190 + 230 + 170 = 2150; 2150 ÷ 10 = 215 min = 3 h 35 min |
| Range | 420 − 150 = 270 min (4 h 30 min) |
| Median | In order: 150, 160, 170, 180, 190, 200, 210, 230, 240, 420. Middle two: (190 + 200) ÷ 2 = 195 min |
| Rule breakers | Bethan (55), Deio (95), Iestyn (60). Gethin (50) kept the rule exactly. |
| Percentage | 3 ÷ 10 × 100 = 30% |
(b) 40 + 55 + 30 + 95 + 45 + 35 + 50 + 25 + 60 + 40 = 475, and 475 ÷ 10 = 47.5, so the arithmetic is right. The conclusion is still misleading. The rule applies to each person, not to the class average: short stretches like Hana’s 25 minutes cancel out long ones like Deio’s 95 minutes. Correct conclusion: 3 of the 10 pupils (30%) broke the break rule, and Deio went 95 minutes, 45 minutes over the limit, so the class does not keep the rule.
(c) Deio’s 420 minutes is the extreme value (an outlier). The median, 195 minutes, is fairer. The mean of 215 is pulled up by Deio: 7 of the 10 pupils are below it, and without Deio the mean would be 1730 ÷ 9 = 192 minutes.
Accurate mean, range and median with working; applies the rule carefully at the boundary (50 is not “more than 50”); checks the representative’s sum before judging the conclusion; explains why an average hides individuals; and chooses the right average because of the outlier. Evidences criteria 3 and 4 and is the main numeracy evidence.
| Setup | What breaks the guidance | Fix |
|---|---|---|
| A. Mali | Seat 45 cm, but her knee-to-floor height is 39 cm, so the seat is 6 cm too high. Feet not flat; the seat edge presses on the back of her knees. | Lower the seat by 6 cm to 39 cm, then lower the desk so her forearms are level. If the seat must stay up, use a 6 cm footrest. |
| B. Owain | Top of the screen 12 cm above eye level, so his neck bends back. The screen faces the window, causing reflections and glare. | Remove the books and lower the screen 12 cm so the top is level with his eyes or just below. Turn the desk so the window is to the side, or close the blinds. |
| C. Rhodri | Screen too close (30 cm, not an arm’s length) and too low, so he hunches and looks down. Mouse too far away, so his arm is stretched. 70 minutes without a break, 20 over the rule. | Raise the laptop on a stand with the top of the screen at eye level, about an arm’s length away; use a separate keyboard and mouse, with the mouse in line with his elbow (within 38 cm). Take a 5 to 10 minute break now: stand, stretch, look into the distance. |
Names every problem against a specific guidance point, including the less obvious ones (glare, mouse reach, break length), and gives fixes with measurements taken from the data. Evidences criterion 5.
5 short, practical points that start with a verb, cover posture, light and breaks, use numbers from the lesson, and include well-being (balancing screen time), not just physical health. Evidences criterion 6.