The significance of inquiry skills is widely acknowledged in science practice across many areas. Unlike experimentation, another form of inquiry skill, observational investigation has been much ignored in science education and thus science assessment. Drawing on reserach development in assessment design, this report provides a design pattern to help assessment designers create tasks assessing students' complex scientific reasoning skills in observational investigation. The design pattern lays out considerations regarding targeted knowledge and skills in this inquiry process, characteristics of situations in which students can evidence that skill, and ways of evaluating their own work with specific examples.