In December, the Illinois state board of education found that 43 percent of students there who took the PARCC English/language arts exam on paper scored proficient or above, compared with 36 percent of students who took the exam online. The state board has not sought to determine the cause of those score differences.

Meanwhile, in Maryland’s 111,000-student Baltimore County schools, district officials found similar differences, then used statistical techniques to isolate the impact of the test format.

They found a strong “mode effect” in numerous grade-subject combinations: Baltimore County middle-grades students who took the paper-based version of the PARCC English/language arts exam, for example, scored almost 14 points higher than students who had equivalent demographic and academic backgrounds but took the computer-based test.

“The differences are significant enough that it makes it hard to make meaningful comparisons between students and [schools] at some grade levels,” said Russell Brown, the district’s chief accountability and performance-management officer. “I think it draws into question the validity of the first year’s results for PARCC.”