Nielsen first reported an “error” in its online counting methodology in November, explaining that its NetView system was failing to recognise some long URLs, which resulted in the technology undercounting the amount of time people spent on certain websites.

The firm later said that the glitch had caused it to undercount the audiences of its top 1,000 websites by 5%, although a fix was due to be put in place for the release of November data.

However, Nielsen has now told clients that data from February 2010 to September 2010 will be “modelled for evaluation”, and any monthly data showing “statistically significant changes” will be restated at the end of Q1 2011. Data for October this year will be fully re-run so Nielsen can provide information for the entire fourth quarter.

The firm told clients: “We believe that full reprocessing for October data and modelling for the data from the prior period is the optimal restatement approach given the historical data available and time required to process. The restatement will allow for trend and year-over-year reporting, thus providing continuity for all clients of Nielsen NetView.”