Ninety employees will be laid off over the next three months at the Nielsen Co.'s Oldsmar global operations center, the company said Thursday. That will bring the total to more than 350 in about an 18-month span. Nielsen laid off at least 117 employees in Oldsmar in late 2007 and about 150 more in 2008. The most recent work force reduction is in response to client cutbacks and the slowing economy, Nielsen spokesman Gary Holmes said in an e-mail. Nielsen brought 1,233 employees with it from Dunedin to Oldsmar in a company relocation that started in 2003. At its height, the Oldsmar site had about 1,800 employees. When the latest round of layoffs is completed, Holmes said, there will be 1,300 employees on site and 300 contractors. The majority of those contractors are from India-based Tata Consultancy Services.