The Use of the DK Option
Inclusion of a DK option in attitude scales can be problematic when a
Likert scale is used because its meaning overlaps with the middle
option. As noted above, depending on the label used, respondents
interpret the middle option differently, including as a response
reflecting an absence of agreement or disagreement with a statement
(substantive response) and as a lack of a response (non-substantive
response) (Klopfer & Madden, 1980). The former interpretation is
consistent with Likert’s intention, a neutral attitude toward the object
reflected by the item, and the latter justifies selection of a DK
response choice.
Illustrative of this potential confusion, researchers found respondents
with clearly defined attitudes who initially provided DK responses to
questions related to those attitudes (Gilljam & Granberg, 1993;
O’Muircheartaigh, Krosnick, & Helic, 2000). Gilljam and Granberg (1993)
found Most (80%) of those subjects indicating DK to the first question
took a position on the subsequent two questions, and the positions they
took were predictive of subsequent behavior in responding to three
questions related to a nuclear referendum. In contrast, Feick (1989)
reported DK responses are provided by truly uninformed respondents as
well as by respondents providing equivocal responses to attitudinal
questions. Schuman and Presser (1996) reported that 30% of respondents
would provide a substantive opinion on a law they know nothing
about if not given an explicit DK option.
While Alwin and Krosnick (1991) found that allowing DK responses had no
effect on the reliability of attitude survey questions, Schuman and
Presser (1996) reported mixed findings with respect to the impact of
inclusion of the DK option on the correlation between items. They
reported that in addition to attitude intensity, education and knowledge
levels also affected the use of the DK response, and those with less
developed or weaker attitudes used the DK option more often. Francis and
Busch (1975) also found increased use of DK with lower educational and
SES levels.
These findings raise the question of what DK means. Does a DK response
mean a lack of access to information required to formulate and provide a
response, uncertainty or a neutral response, or unwillingness to provide
a response? Is it interpreted differently by informed and uninformed
respondents? Without disentangling the meaning of the DK and the
middle-option response, the validity of the research findings is
threatened.
In a study with University students (Lam, Allen, & Green, 2011), the
authors examined how respondents with (informed) and without
(uninformed) the necessary knowledge to form an opinion utilized Neutral
and DK response options when presented alone or together and when
placement of the option varied—placed as a mid-point or at the end of
the scale. Five findings are notable. First, both informed and
uninformed respondents used the DK and Neutral anchors interchangeably,
implying a blurred semantic distinction between the two terms as
suggested by the literature.
Second, when both the DK and Neutral options were provided ,
uninformed respondents chose the DK more often than the Neutral, and,
the proportion of DK response for the uninformed group was much higher
than for the informed group. This finding suggests higher validity of
responses when both DK and Neutral options are provided than when only
one of the two options is provided.
Third, when both the DK and Neutral options were provided , both
informed and uninformed respondents selected the DK option more than the
Neutral option when either one of these two options was located in the
middle of the rating scale. These results are consistent with earlier
findings of Krosnick (1999) and Schuman and Presser (1996) who observed
greater use of the nonresponse option when located earlier in the
response scale; i.e., a primacy effect for options presented in a
written format. The meaning of the DK response option is apparent when
it is placed physically as the mid-point, and it is chosen as the
appropriate response. This finding suggests a placement effect.
Fourth, when offered either a DK or Neutral option but not
both, when presented in the middle of the rating scale, both
informed and uninformed participants chose that option more often than
when presented at the end of the scale. This finding further supports a
placement effect.
Fifth, if either a DK or Neutral option was provided, for the
uninformed respondents, when the Neutral option was placed at the end of
the response scale (with an extra space separating it from the other
rating options), they were more likely to interpret it as a
non-substantive response option than when the option was placed in the
middle of the scale. However, the informed respondents tended to choose
either the neutral or DK option more often when placed in the middle
rather than at the end of the response scale. This observation implies
that informed respondents may rely on placement as a satisficing
strategy when they do have the necessary knowledge to respond to an
item, but perhaps lack sufficient attitude strength to reply
meaningfully (Schuman & Presser, 1996). The uninformed respondents
lacked the knowledge to provide substantive responses, meaning they did
not need a satisficing strategy. Instead, they simply looked for a way
to indicate an absence of attitude or evaluative judgment or opinion by
selecting a no response option.