The "Cannot Decide" Option in Thurstone-Type Attitude Scales

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Abstract
Sociology students were administered two Thurstone-type attitude scales (Capital Punishment and Sunday Observance), each under two conditions (with and without the option of responding"?" for "Cannot Decide"), and a measure of Ambiguity Tolerance. When available, "?" was used by a slight majority of subjects on both scales. The use of "?" was not found to be related to scores on the measure of ambiguity tolerance. Correlations between "?" and "no?" administrations were significant, and significantly stronger for Capital Punishment than for Sunday Observance, but means did not differ significantly between conditions, and mean differences did not differ significantly between scales. More use of "?" appeared in the Capital Punishment scale than in the Sunday Observance scale but "?" substituted fairly evenly for "Agree" and "Disagree" in Capital Punishment, while in Sunday Observance "?" substituted mainly for "Disagree." Allowance for "?" is recommended. Observed differences are discussed in terms of assimilation-contrast theory, mainly the constructs of statement-attitude congruity, involvement with the attitude object, strength of the attitude, and development of the attitude.