Both PIRLS and PISA assess aspects of reading skills, but they differ in terms of whom they assess and what they assess.
PIRLS assesses 4th-graders and is designed to reflect the curriculum of participating countries. PIRLS asks students to read two texts—either two literary texts (narrative fiction, generally drawn from children's books), two informational texts (typically excerpts from biographies, step-by-step instructions, or scientific or non-fiction materials), or one of each type. It then asks students about a dozen questions (both multiple-choice and open-ended "constructed response") about the texts that range from identifying the place, time, and actions of the main characters or events to interpreting how characters might feel, why events occurred, or what the passage means overall (e.g., does the story teach a lesson?).14
PISA assesses 15-year-old students and does not explicitly focus on curricular outcomes; rather it focuses on cognitive skills and the application of reading to problems within a real-life context. Thus it presents students with a range of texts that they are likely to encounter as young adults, such as excerpts from government forms, brochures, newspaper articles, instruction manuals, books, and magazines. For each text, it then usually asks each student 3–5 questions (both multiple choice and constructed response) to measure the extent to which students can retrieve information, interpret a text, reflect on a text, and evaluate its author's rhetorical choices.15 In years when PISA focuses on reading, students receive between 12 and 24 reading texts (depending on the particular cluster of items in their particular test booklet); when PISA focuses on mathematics or science, students receive about 7 reading texts.
14 Examples of PIRLS items can be viewed at http://nces.ed.gov/pubs2008/2008017_2.pdf.
15 Examples of PISA reading items can be viewed at http://www.oecd.org/dataoecd/30/17/39703267.pdf, pages 288 to 291.