Were all the signs placed in one neighbourhood? This could partly invalidate the test, since there could be an effect of discussing the issue with neighbours and changing opinions as a result of discussions, not solely as a result of a consistency effect. The study should have avoided this, choosing houseowners far from each other. Was it the case?
Maybe seeing the little signs on their neighbors’ yards made them believe signs were acceptable in their neighborhood.
Were all the signs placed in one neighbourhood? This could partly invalidate the test, since there could be an effect of discussing the issue with neighbours and changing opinions as a result of discussions, not solely as a result of a consistency effect. The study should have avoided this, choosing houseowners far from each other. Was it the case?