Self-reported data in environmental health studies: mail vs. web-based surveys

Table 1 Number of responses and its corresponding response rate obtained at each data collection phase (i.e. after the first time participants were contacted, after participants received at least one reminder letter and for the overall study)

Region	Number of mailed letters ^a	Number of mailed reminders ^a	Overall responses		Responses after first contact		Responses after reminder		p-value ^e
Region	Number of mailed letters ^a	Number of mailed reminders ^a	Number of respondents	Response rate (%) ^b	Number of respondents	Response rate (%) ^c	Number of respondents	Response rate (%) ^d	p-value ^e
Anholt	79	59	22	27.8	20	25.3	2	3.4	< 0.001
Keldsnor	165	134	61	37.0	31	18.8	30	22.4
Lindet	645	526	201	31.2	119	18.4	82	15.6
Sundeved	2215	1736	782	35.3	479	21.6	303	17.5
Total	3104	2455	1066 ^f	34.3	649	20.9	417	17.0

^a Respondents estimated to be ineligible (i.e. returned letters due to invalid address and refusals due to e.g. the fact that the house is not used on a daily basis (i.e. vacation house) and rare cases of mental sickness informed by a health care assistant) are excluded from this number. In total, 75 households were considered ineligible
^b Response rate was calculated by dividing the total number of respondents by the number of mailed letters
^c Response rate was calculated by dividing the number of respondents after the first contact by the number of mailed letters
^d Response rate was calculated by dividing the number of respondents after a reminder letter by the number of mailed reminders
^e Pearson’s Chi-squared test of proportions was used to calculate the difference between the response rates for the two contact points
^f Besides these surveys, 13 other responses were received without the identification code and two individuals have answered both the mail and the Web version of the questionnaire. These were not considered in this study and therefore, are not considered in this analysis

ISSN: 1471-2288