Reading proficiency among New York City public school students in grades three through eight fell six percentage points last school year, with third graders dropping nearly 14 points, according to preliminary state test data released Friday.

About half of city students in those grades scored proficient in reading, Chalkbeat reported. The decline nearly erases a seven percentage point increase the year before, which officials had cited as evidence that the city's literacy overhaul was working.

The drops were steepest in the early grades. Fourth grade scores fell close to 6 points and fifth grade fell 13 points, according to Chalkbeat. Math results were essentially flat, with 57 percent of students proficient.

Statewide, reading proficiency fell 5 percentage points, with 48 percent of students meeting the standard. State math proficiency rose 2 points to 57 percent.

The city education department described the year-to-year fluctuation as unprecedented and said it would conduct a deeper analysis, NY1 reported. Schools Chancellor Kamar Samuels said the goal remains universal literacy and that the city will continue with NYC Reads and its science of reading approach, according to Gothamist.

Testing experts told Chalkbeat the swings are unusual, since state test results normally shift gradually. Andrew Ho, a Harvard professor who serves on a panel advising New York on its assessment system, said such movement is uncommon but not unprecedented, and that the true gain and decline are probably both smaller than the numbers suggest.

Aaron Pallas of Teachers College said the pattern raises concern about something unusual in last year's exam and how it was scored, telling Chalkbeat that without that year's data the results would look fairly stable.

The state lowered the number of questions students needed to answer correctly to be counted proficient in third, fourth and sixth grade reading last year, precisely the grades where this year's decline is steepest. State officials say such adjustments are routine and are intended to keep exams equivalent from year to year.

Individual student results go home to families Monday.