Fukushima Prefecture and the National Institute of Radiological Sciences have said they erroneously estimated the radiation exposure of 16,118 people in a survey covering the first four months after the outbreak of the March 2011 disaster at the Fukushima No. 1 nuclear plant.

Among the roughly 420,000 people authorities have so far finished compiling data on, recalculations show 12,469 received higher doses and 3,649 lower doses than previously estimated.

The margins for revisions range from plus 0.4 millisievert to minus 0.2 millisievert. As a result of the revisions, it was learned that some people were exposed to more than 1 millisievert — the annual limit set by the government for ordinary citizens.

People polled were asked to answer in detail where they were between March 12 and July 11, 2011.

Based on their whereabouts, the institute estimated their cumulative amount of external exposure by adding up daily radiation levels measured at their locations over the four months.

Used as reference were actual radiation readings at a number of monitoring posts in the prefecture as well as projections of the spread of radioactive substances by the SPEEDI computer simulation system.

But in some cases, the dates in the survey failed to match those in the reference data.