A telephone survey of factors affecting willingness to participate in health research surveys

BMC Public Health

Table 1 Study population and participation rates by study sample groups

	Complete study population
	General population sample				Case–control study	Cohort study	Total study population
	Landline			Mobile telephone	Case–control study	Cohort study
	Letter sent	No letter	All landline	No letter	Letter sent	Letter sent
Study population approached n	1806	2194	4000	3000	200	600	7800
Non-contact/phone number unusable^a n	391	1123	1514	2005	33	77	3629
Eligible population^b n	1415	1071	2486	995	167	523	4171
Refusals^c	761	631	1372	540	14	122	2068
Completed interviews n	654	440	1094	455	153	401	2103
Participation rate %	46.2	41.1	44.0	45.7	91.6	76.7	50.4

^aIncludes no answer, engaged, answering machine, those under 18, those with language difficulty and too frail/old/ill
^bExcludes non-contacts
^cIncludes those who actively declined or claimed to have already done the survey, or made but did not keep an appointment, or terminated during the survey, or said that they did not know the named person or who asked to be removed from the list

ISSN: 1471-2458