Theoretical Reflection

the DK option is a generic category for respondents who are not able or willing to give an answer, i.e. a substantive answer, to a closed-ended survey question. in this experiment the aim is to explore and analyze what the level of (item) nonresponse is for four different versions of a questionnaire and whether significant differences can be found both with respect to the way this DK option is offered and as regards the topics that are being addressed in the survey questions. All survey questions included in the experiment concern attitudes.

the debate on non-substantive answers and how to handle this problem is old but not yet concluded. there is still disagreement in the literature about how best to handle non-substantive answers, i.e. item nonresponse, for opinion and attitude questions(see e.g. Gilljam & Granberg, 1993; heerwegh & loosveldt, 2008; Krosnick et al., 2002; leigh & Martin, 1987). in practice, respondents are usually encouraged to give their opinion: “the typical practice - what we shall call the standard question

form - is not to include a DK alternative as part of a question” (schuman & Presser,

199, p. 113). schuman and Presser (199) state that only spontaneous DK answers are registered in this standard practice; an explicit DK is not part of the response categories. the reason is that researchers want to increase their item response rate and collect as many substantive responses as possible.

some scholars differentiate between reasons for using a non-substantive response option, e.g. lack of information, or a polite refusal to a difficult or sensitive 1 A pilot experiment was conducted among high school students (see Van de Maat, 2009).

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question (bradburn et al., 2004, p. 353), but such a distinction is not made here. the question why respondents use a DK option is relevant but empirically very difficult to analyze. More importantly, the aim in this study is to look at levels of item nonresponse and the resulting picture of public opinion regardless of the reasons for these non-substantive answers. the quantity and not the quality of item nonresponse is examined here. in this experiment the aim is to analyze what the level of item nonresponse is for each version of questionnaire and whether significant differences can be observed. People may give a DK answer for various reasons, but here the aim is first and foremost to establish general levels of item nonresponse and not to differentiate. the additional analysis of response time in this chapter may, however, give some indication why respondents use a DK option. 5.3 Hypotheses

in chapter 3 several hypotheses were developed. these hypotheses are summarized in table 5.1.

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Table 5.1: Hypotheses

5.4 Data and Methods

for the first experiment, the liss Panel, longitudinal internet studies for the social sciences17_{, was used. this panel forms the core of the Measurement and} experimentation in the social sciences (Mess) project; it was funded between 2007 and 2014 by the netherlands foundation for scientific Research (nWO) to give 17 www.lissdata.nl for a description of the panel and examples of previous experiments which have used

this panel. the liss panel is administered by centeRdata (tilburg university, the netherlands). the author would like to thank the liss panel for their support and assistance in setting up and execut- ing the survey experiment.

Question design Missing data Question content Response categories Response time break-offs h1a h1b h2a h2b h3a h3b h4a h4b h4c h5 h

h1a: the more explicit a non-substantive response option is

presented, the more item nonresponse will be measured

A filter question results in more item nonresponse than an explicit DK option

(based on MAR) An increase of the level of missing data does not affect the distribution of opinions

(based on nMAR) An increase of the level of missing data results in a different distribution of opinions

if the topic of a survey question is related to a major political dimension, then the item nonresponse is lower compared to a survey question that is not related to such a dimension

the item nonresponse for questions about foreign policy issues is higher than for questions about issues related to the core dimensions the more substantive response categories are offered, the lower the item nonresponse rate

A midpoint in the absence of a non-substantive response option results in more use of this midpoint option than when a non- substantive response option is offered

A midpoint combined with a non-substantive response option results in less item nonresponse as compared to offering no midpoint category

the more explicit the DK option is presented, the less response time will be registered

When respondents are forced to answer survey questions, the number of break-offs is higher than when a non-substantive response option is available

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researchers the opportunity to collect data via surveys. On average, 73 percent of the panel members complete the questionnaires. see section 4.3 for a more extensive description of the liss Panel characteristics.

the large n of the liss panel allows to divide respondents into subgroups which were subjected to one particular question design and stimulus (see table 5.2 below) and to subsequently compare the data. the full panel was targeted, with random selection of the four distinct groups for the various treatments.

the general instruction of the questionnaire read that the respondents were asked to give their opinion on issues in the poll or survey. scales for self-placement were placed horizontally while the response categories of the other options were ranked vertically. the experiment was carried out between february 1st and february 29th2012; respondents were given a month to complete the questionnaire.

table 5.2 contains descriptive statistics of the sample and subgroups to check the comparability of the subgroups and the representativeness of the sample. Overall the subgroups did not differ from each other. the only exception is a small but statistically significant difference between subgroups in terms of gender – see table c.1. in the appendix. On average 47 percent of the respondents was male, but in subgroup 3 they made up 50 percent of the subsample. the average age of the respondents was 49.9 years with no significant difference between subgroups. in terms of the highest level of education, the subgroups also were similar with no significant difference between groups. the average monthly individual income of the household member filling in the survey was 1,495 euro. there was no significant difference in income found between subgroups.

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Table 5.2: Descriptive Statistics Subgroups LISS Panel

* Any pair-wise comparison with variant 3 is significant at the 0.05 level Gender: percentage of the sample that is male or female.

Age: Average age in years, with respondents being 1 and older.

education: highest level of education completed, recoded into cbs categories. income: Average individual income of the household member.

the overall similarity of the subgroups is important to make comparisons between the variants. the comparability of the sample as a whole with the population is less important, since it is an experiment with a between-subjects-design and the key conclusions are drawn after comparing the four subgroups. in order to be able to generalize the findings to the general population, however, the sample should not differ too much from this population. comparing the sample with the population is problematic, however, due to the lack of population data. statistics netherlands (cbs)18 _{offers information about the population, but these figures are not directly} comparable to the sample19_{. nevertheless, based on the random sampling method,} it is assumed that the sample is comparable to the population and that generalization to this population is possible.

18 statline.cbs.nl with data from 2011.

19 According to cbs, 49.49 percent of the population is male. the average age of the population is 40.3

years, but this includes people who are younger than 1 and these are not included in the liss Panel.

% of the population finished basisonderwijs, 20% VMbO, 42% hAVO/VWO & MbO, 21% hbO and 11% WO. finally, the average income for the population as a whole is 22,300 euro a year including social security payments, welfare, wages, holiday pay and bonuses.

Gender* Male female

Age (years) Mean

sD education basisonderwijs VMbO havo/VWO MbO hbO WO income Mean (euros/month) sD N Total 47% 53% 49.9 17.3 10% 26% 11% 23% 23% 8% 1495 3149 1. Double Explicit DK 45% 55% 49.7 17.3 10% 2% 11% 22% 22% 9% 142 1030 1468 2. Single Explicit DK 4% 54% 50 17.3 10% 25% 12% 23% 23% 8% 1494 2937 1464 3. Single Implicit DK 50% 50% 49.7 17.3 9% 2% 11% 24% 23% 8% 109 515 1375 4. Forced choice 4% 54% 49.8 17.0 10% 2% 11% 22% 23% 8% 145 208 1421

5

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Table 5.3: Response Rates

table 5.3 presents some characteristics of the experiment, including the unit (non)response rate and the average item nonresponse rate of each variant of the questionnaire. Out of the 7,517 individual members of the liss panel, 5,728 participated in the experiment, i.e. an average unit response rate of 7 percent, with rates ranging from 73 to 78 percent for the individual variants. All respondents answered 14 items relevant for this experiment in their respective variant of the questionnaire.

the questions covered a range of topics. five general themes were addressed and each theme included three questions of which at least one question came from existing research – see section 4.4 for more information about issue selection. the aim of this part of the experiment is to compare between themes, but also within themes. for the socio-economic theme, for example, there are differences expected between the general ‘income differences’ question versus the at that time discussed particular issue ‘old-age pension’.

the stimulus in the survey experiment is a ‘Don’t Know’ (DK) response category which was offered (1) explicitly in both the question wording and as a response alternative; (2) not mentioned in the question wording but explicitly offered as a response alternative; (3) not mentioned in the question wording nor as an explicit response alternative, but with the possibility to skip the question without giving a substantive answer; and finally (4) not mentioned in the question wording nor as an explicit response alternative, and with no possibility to skip the question without giving any substantive answer20_{. this final forced choice variant is often used in} 20 the ways the DK option was offered would be labelled as DK encouraging and DK neutral by luskin and bullock. in order to examine the effects of several DK designs on the measurement of political knowledge, luskin and bullock (2011) applied three ways of offering a DK option: 1) DK discouraging with no explicit DK option and a probe to get more substantive answers; 2) DK encouraging with an

instruction to give a DK answer when unsure and an explicit DK option; and 3) DK neutral. in this survey experiment, no variant was used where the respondent was probed for a response, since this is uncommon in standard web surveys.

Variant 1. Double explicit DK 2. single explicit DK 3. single implicit DK 4. no DK, forced choice Total Number of Respondents 148 144 1375 1421 5728 Response Rate – Unit 78% 78% 73% 7% 76% Number of Items in Questionnaire 14 14 14 14 14 Average Item Nonresponse 1% 14% 1% X

85 ‘interactive self-administered surveys’ (Derouvray & couper, 2002). the number and choice of answer categories was replicated from the original questions and not altered in any way; only the DK option was manipulated. the DK option was the only available non-substantive response option; no comparison was made between the use of either Don’t Know or no Opinion as a means to register non-substantive answers.

table 5.4 shows the four versions of the questions/questionnaire (see Appendix Ai for the full questionnaires). And by way of illustration, figure 5.1 shows the variations of a survey question, depending on the variant offered to the respondent. the parts in italics vary.

Table 5.4: Overview of DK Alternatives a)

a) in the single implicit DK alternative there is no explicit DK response category, but it is possible to skip the question without giving any answer; in the no DK alternative there is no DK response possible and it is not possible to continue with the questionnaire without giving an answer to the question.

type of question 1. Double explicit DK 2. single explicit DK 3. single implicit DK 4. no DK DK alternative offered? in question wording + - - - as response alternative + + ± - 57

Table 5.4 shows the four versions of the questions/questionnaire (see Appendix AI for the full questionnaires). And by way of illustration, Figure 5.1 shows the variations of a survey question, depending on the variant offered to the respondent. The parts in italics vary.

Table 5.4: Overview of DK Alternatives a)

DK alternative offered?

Type of question in question wording as response alternative

1. Double explicit DK + +

2. Single explicit DK - +

3. Single implicit DK - ±

4. No DK - -

a) _{In the single implicit DK alternative there is no explicit DK response category, but it is possible to skip the question} without giving any answer; in the No DK alternative there is no DK response possible and it is not

possible to continue with the questionnaire without giving an answer to the question.

5.5 Results

In this section, the results are presented of the survey experiment in which a DK option was manipulated. The presentation of results is structured according to the hypotheses, starting with an analysis of item nonresponse. This analysis includes both the number of times the DK option was used and the registered time to answer the question. After that, the relation between missing data and the overall distribution of opinions is examined, followed by an inventory of differences between question content and number of response categories.

5.5.1 Item Nonresponse

‘Some people think that euthanasia should be forbidden by law. Others feel that a doctor should always be allowed to end a life, if the patient makes that request. Of course, there are also people whose opinions lie somewhere in between. Suppose that the people (and parties) who think that euthanasia should be forbidden are at the beginning of this line (at number 1), and the people (and parties) who feel that a doctor should always be allowed to end a life upon a patient's request are at the end of the line (at number 7).

Where would you place yourself on the line?’ If you have no idea at all which position a party has, then please feel free to say so.

1. Euthanasia should be forbidden. 2 - 6

7. A doctor should always be allowed to end a life upon a patient’s request.

99. Don’t Know

Figure 5.1: Survey Question Variations

Figure 5.1: Survey Question Variations

8

5.5 Results

in this section, the results are presented of the survey experiment in which a DK option was manipulated. the presentation of results is structured according to the hypotheses, starting with an analysis of item nonresponse. this analysis includes both the number of times the DK option was used and the registered time to answer the question. After that, the relation between missing data and the overall distribution of opinions is examined, followed by an inventory of differences between question content and number of response categories.

5.5.1 Item Nonresponse

figure 5.2 shows the average item nonresponse for the complete questionnaire for the four question variants.

Figure 5.2: Average Item Nonresponse Rate (in Percentages)

figure 5.2 shows the average item nonresponse rate of the four questionnaire variants; all respondents answered 14 items and the variants only differed with respect to their DK option. there is a substantial difference between variants explicitly offering a DK option, i.e. variant 1 and 2, and variant 3 which made item nonresponse possible by skipping the questions, but did not offer this option explicitly as part of the question. Despite the explicit instruction at the start of variant 3 that skipping questions was possible, this option was not used often. the instruction was given because the respondents of the liss panel are usually not

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Figure 5.2 shows the average item nonresponse for the complete questionnaire for the four question variants.

Figure 5.2: Average Item Nonresponse Rate (in Percentages)

Figure 5.2 shows the average item nonresponse rate of the four questionnaire variants; all respondents answered 14 items and the variants only differed with respect to their DK option. There is a substantial difference between variants explicitly offering a DK option, i.e. variant 1 and 2, and variant 3 which made item nonresponse possible by skipping the questions, but did not offer this option explicitly as part of the question. Despite the explicit instruction at the start of variant 3 that skipping questions was possible, this option was not used often. The instruction was given because the respondents of the LISS panel are usually not given this option and would therefore probably not consider it an option. This could be the reason the respondents did not skip questions often, as they might not have been aware the option existed. The instruction was given in the introduction of the questionnaire.

In variant 1 respondents were twice pointed to the possibility of using a DK option, by explicitly mentioning the DK option in the question and as a response category, as opposed to only a DK response category in variant 2. The difference in average item nonresponse rate is 2 percent between variant 1 (16 percent) and 2 (14 percent). To test whether the difference in item nonresponse rate was statistically different, a t-test was executed; the difference is small

but statistically significant at the .001 level21_{. The main difference exists, however, between}

both these explicit DK variants and the implicit or forced choice variants 3 and 4. When respondents could skip a question but were not reminded of that option (variant 3), the item nonresponse rate dropped to 1 percent. These overall results show that the way a DK option is offered has a significant and substantial influence on item nonresponse, as stated in H1a. Offering a DK option more explicitly results in more item nonresponse; note that offering such an option explicitly once already makes a major difference.

21 _{In addition to the t-test, a negative binomial regression was performed with the number of non-substantive} answers as a dependent variable, to account for the fact that the data are not normally distributed. The results were almost identical to the t-test.

0 1 14 16 0 2 4 6 8 10 12 14 16 18 4. Forced choice 3. Single implicit DK 2. Single explicit DK 1. Double explicit DK

87 given this option and would therefore probably not consider it an option. this could be the reason the respondents did not skip questions often, as they might not have been aware the option existed. the instruction was given in the introduction