A total of 10,350 children and young people belong to the two Leicester cohorts:

• The first cohort has 1,650 young people whose parents received the first questionnaire in 1990 when their children were between 0 and 5 years old.
• The second cohort has 8,700 children. Their parents received the first questionnaire in 1998 when their children were aged 1 to 4 years.

The addresses of these families were chosen at random from all families living in Leicestershire. We did not know in advance which children would have breathing problems. In fact, most participants in this survey are very healthy!

The Leicestershire and Rutland Research Ethics Committee approved the study, because it contributes to improving child health, is carried out carefully, and data confidentiality is guaranteed.

Most families have filled in follow-up questionnaires at regular intervals, telling us how the children were getting on. Some families answered only some of the questionnaires. Although we are extremely grateful for people who participate regularly, any contribution is valuable, even if it is only occasional.