Millions of laypersons need more medical information than they are customarily provided during their doctor's visit. Health websites can help fill this knowledge gap, but the text is believed to be too difficult to understand for many laypersons. To help write text that is not perceived as too difficult and leads to better comprehension (actual difficulty), we study how linguistic structures influence text difficulty. Since perceived difficulty has been shown to be a barrier to self-education, evaluating perceived difficulty is an essential first step to take. In this study, we evaluated the impact of noun phrase complexity and function word density in four sentence structures (active, passive, sentential or extraposed subject). Complex noun phrases significantly increased perceived difficulty while using more function words significantly decreased perceived difficulty. Furthermore, laypersons judge text differently when they perform the evaluation on behalf of themselves compared to evaluating on behalf of other readers.