dcf.cymru stimulus sheet · Maths · Year 11

Broadband speeds in Bryn Eira and Cwm Derwen: the community council survey

The two villages, the community council, the people and every figure in Sections 2 to 6 are invented. The broadband facts in Section 1 are real.

1. Background: real facts about broadband

Download speed is measured in megabits per second (Mbps). A higher number means a faster connection.
Many homes in Wales get fibre to the cabinet broadband. A fibre-optic cable runs to a green street cabinet, and the old copper telephone line carries the signal from the cabinet to each home. The longer the copper line, the slower the speed, so homes far from the cabinet usually get slower speeds.
Ofcom, the UK communications regulator, calls a download speed of 30 Mbps or more “superfast”.
Under the broadband Universal Service Obligation, a home that cannot get a download speed of at least 10 Mbps (and an upload speed of at least 1 Mbps) may be able to request a better connection.
A speed test run over Wi-Fi often gives a lower result than one run on a computer plugged into the router with a cable, because walls and distance from the router slow Wi-Fi down.

2. The two villages

Village	Homes	Layout
Cwm Derwen	297	A compact village. The broadband cabinet is in the square, and most homes are within 600 m of it.
Bryn Eira	214	Homes are spread along 3 km of valley road. The cabinet is at the bottom of the valley, by the main road, so some homes are more than 2.5 km from it.

3. The hypothesis and a claim

Councillor Rhys Bevan wants evidence for a letter to the broadband network operator. He writes the hypothesis:

“Homes in Cwm Derwen get faster and more consistent download speeds than homes in Bryn Eira.”

A Bryn Eira resident posted on the council’s online noticeboard:

“Nobody in Bryn Eira gets a decent speed. We are all stuck below 30.”

4. How the data were collected

The council put a letter through every door in both villages asking for volunteers.
Each volunteer ran one free online speed test between 20:00 and 21:00 on Tuesday 9 June 2026, and typed the download speed into the council’s online form. The form also asked whether the test was run over Wi-Fi or with a cable.
80 homes in each village took part.

5. The results

Table 1: Bryn Eira (80 homes)

Download speed, s (Mbps)	Number of homes
0 < s ≤ 10	10
10 < s ≤ 20	12
20 < s ≤ 30	14
30 < s ≤ 40	16
40 < s ≤ 50	11
50 < s ≤ 60	8
60 < s ≤ 70	6
70 < s ≤ 80	3

Slowest home: 2.8 Mbps. Fastest home: 76.4 Mbps.

Table 2: Cwm Derwen (80 homes)

Download speed, s (Mbps)	Number of homes	Cumulative frequency
0 < s ≤ 10	0	s ≤ 10: 0
10 < s ≤ 20	3	s ≤ 20: 3
20 < s ≤ 30	13	s ≤ 30: 16
30 < s ≤ 40	26	s ≤ 40: 42
40 < s ≤ 50	24	s ≤ 50: 66
50 < s ≤ 60	10	s ≤ 60: 76
60 < s ≤ 70	4	s ≤ 70: 80
70 < s ≤ 80	0	s ≤ 80: 80

The council estimated these values from a cumulative frequency graph for Cwm Derwen. The slowest and fastest homes come from the raw data.

Slowest	Lower quartile	Median	Upper quartile	Fastest
11.2 Mbps	31.5 Mbps	39.2 Mbps	47.5 Mbps	66.8 Mbps

6. How the tests were run

The council worked these out from the raw data.

Village	Tests over Wi-Fi	Tests with a cable	Median speed, Wi-Fi tests	Median speed, cable tests
Bryn Eira	49	31	29.7 Mbps	36.2 Mbps
Cwm Derwen	34	46	38.3 Mbps	39.4 Mbps

All 160 results are in broadband-bryn-eira-cwm-derwen-speeds.csv, one row per home, if you have a computer: home ID, village, download speed in Mbps, connection (Wi-Fi or Cable) and the time of the test.