The BES internet panel data is collected by YouGov using an online sample of YouGov panel members.
In addition, after each General Election we also conduct a face-to-face survey using a representative sample of the general population. The 2015 and 2017 face-to-face fieldwork was overseen by GfK, while the 2019 fieldwork was overseen by Ipsos MORI, each time surveying the original 2015 sample.
The 2019 study was designed as a face-to-face in-home survey administered by an interviewer via Computer Assisted Personal Interviewing (CAPI). Fieldwork commenced under the original design but was halted in mid-March 2020, as a result of the coronavirus pandemic. A second phase of research was implemented, following a push-to-web with a postal follow-up approach with the questionnaire self-administered by respondents, either online via Computer Assisted Web Interviewing (CAWI) or on paper (PAPI).
More details are available in the technical report for each survey, included alongside the data on the cross-sectional data page.