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Beukenhorst and Wetzels (2009) describe a mixed‐mode experiment conducted by Statistics Netherlands. They used the Dutch Safety Monitor for this experiment. This survey asks questions about feelings of security, quality of life, and level of crime experienced. The sample for this survey was selected from the Dutch population register. All sampled persons received a letter in which they were asked to complete the survey questionnaire on the Internet. The letter also included a postcard that could be used to request a paper questionnaire. Two reminders were sent to those that did not respond by web or mail. If still no response was obtained, nonrespondents were approached by means of CATI, if a listed telephone number was available. If not, these nonrespondents were approached by CAPI.

To be able to compare this four‐mode survey with a traditional survey, also a two‐mode survey was conducted for an independent sample. Sampled persons were approached by CATI if their telephone number was listed in the directory, and otherwise they were approached by CAPI.

The response rate for four‐mode survey turned out to be 59.7%. The response rate for the two‐mode survey was higher. So, introducing more modes did not increase the overall response rate. However, more than half of the response (58%) in the four‐mode survey was obtained with a self‐administered mode of data collection (web or paper). Therefore, the costs of the survey were much lower. Interviewers were deployed in only 42% of the cases. For more detail, see Beukenhorst and Wetzels (2009) or Bethlehem, Cobben, and Schouten (2011).

A special case of mixed‐mode data collection is related to the increasing diffusion of mobile phones and smartphones. When an invitation e‐mail is sent, the questionnaire might be received either on a computer or a mobile phone or a smartphone. The interviewee could complete the web questionnaire using either the mobile device or the computer. Thus, it is better to talk about mobile web surveys rather than web survey. A recommendation is to run web surveys that are fully adapt for smartphones. Therefore, in presenting methods for web surveys, comments about the adaption for smartphone surveys will be discussed all along the chapters of this handbook. Some penetration data allow for understanding how the situation differs across the countries. The coverage of telephone directories, of Internet, and of mobile cells provides the feeling of the need to adopt a mixed‐mode approach. A World Bank study reports that, in the 2018, Euro area fixed telephone subscription for 100 people is 44.4, mobile 122.6 and Internet 83.8. Table 1.2 shows the same indicators by country. Only some countries are shown in the table since the objective is just to evidence that there is a relevant difference across countries.

Table 1.2 Penetration of fixed and mobile phone and of Internet (year 2018)

Source: Data from International Telecommunication Union. World Telecommunication/ICT.

Country	Fixed telephone subscription (% of inhabitants)	Mobile cellular subscription (% of inhabitants)	% of individuals using the Internet
Austria	42	125	88.0
Denmark	19	125	97.6
Finland	6	132	88.9
France	59	108	82.0
Germany	52	129	89.7
Greece	47	116	72.9
Italy	34	137	74.4
The Netherlands	35	124	94.7
Norway	11	107	96.5
Portugal	50	115	74.7
Romania	19	116	70.7
Slovenia	33	118	79.7
Spain	40	116	86.1
Sweden	24	125	92.1
Switzerland	39	130	90.0

Note that Internet users are individuals who have used the Internet (from any location) in the last 12 months. Internet can be used via a computer, mobile phone, personal digital assistant, games machine, digital TV, etc. Fixed telephone subscriptions refer to the sum of active number of analogue fixed telephone lines, voice‐over‐IP (VoIP) subscriptions, fixed wireless local loop (WLL) subscriptions, ISDN voice‐channel equivalents, and fixed public payphones. Mobile cellular telephone subscriptions are subscriptions to a public mobile telephone service that provide access to the PSTN using cellular technology.