S4C's Reaction Panel is conducted by TRP.

There are 1,400 people on the panel consisting of Welsh and non Welsh speakers, with the data weighted to be representative of Welsh speakers and Welsh people overall in Wales in terms of geography, age, social class and sex.

Every week, each member receives a programme diary and occasionally a questionnaire. The programme diary contains all the programmes within that week for S4C. The member awards a mark out of 10 for each programme watched in the diary, resulting in AI's [Appreciation Indices] for each of these programmes. An AI is a score out of 100 and S4C's programme scores are compared with that of the genre. It is also possible to compare channel scores at a general level.

The questionnaire content varies each week with questions about television programmes as well as questions about general issues.