Too many subquestions in a block and too many questions on one page
It is known that respondents quit the survey when presented with a block of questions where they must, for example, specify how much they agree with a number of statements.
A single block should contain no more than 8 to 10 questions. Any additional questions should then be included in a second block that contains a new (shorter) question text. With very few exceptions, a single survey page (including the introduction page) should include only as many questions as can (at the same time) be displayed on an average computer screen with an average display resolution. In addition, the "next page" button should be visible without the need for page scrolling.
A greater number of questions (or even all of them) should be put on a single page only in special cases and with very good reason because this forces the user to navigate the questionnaire by scrolling. The same applies to questions that are displayed only when a condition is met – unless they are very brief, such questions should be placed on a new survey page.