An important objective in survey question design is to write clear questions that respondents find easy to understand and to answer. This contribution identifies the factors that influence question clarity. Theoretical and empirical evidence from psycholinguistics suggests that specific text features (e.g., low-frequency words (LFRWs), left-embedded syntax) cause comprehension difficulties and impose a high cognitive burden on respondents. To examine the effect of seven different text features on question clarity, an online experiment was conducted in which well-formulated questions were compared to suboptimal counterparts. The cognitive burden of the questions was assessed with response times. Data quality was compared in terms of drop-out rates and survey satisficing behaviour. The results show that at least six of the text features are relevant for the clarity of a question. We provide a detailed explanation of these text features and advise survey designers to avoid them when crafting questions.