Tasks used for assessment can range from short pithy questions to focus on a concept, such as "What is a sample?" or "What is variation?", to multi-part tasks that build in difficulty to discover students’ optimal level of performance (e.g. Comparing Two Groups).
It is important to consider the marking scheme while a task is developed and this is likely to be associated with an assessment rubric.
A bank of survey or test items suitable for middle school is found in appendix C (page 31) to the paper on Statistical Literacy (286 KB PDF from the International Association for Statistical Education website). You can also use the covariation assessment to determine students' prior learning.
Many brief tasks, often multiple-choice or computationally based, are available in textbooks or through the Australian NAPLAN website.
