A survey of U.S. hiring managers found more than a third indicated they had positions unfilled for 12 weeks or more, CareerBuilder said. In a survey of more than 2,000 hiring managers conducted May 14 through June 5, CareerBuilder said 35 percent indicated they had job openings for which finding a candidate with the required skill set was proving to be difficult. \"Two in five employers (41 percent) reported that they continuously recruit throughout the year, so that they have candidates in their pipeline in case a position opens up down the road,\" said Brent Rasmussen, president of CareerBuilder North America in a statement. Rasmussen predicted the \"skills gap ... [would] become even more pronounced in the years to come.\" The answer to that problem is \"formal education and on-the-job training,\" he said. The problem of filling a job position can be more pronounced in specialized jobs that grow so quickly that schools and training centers are not keeping up with increased demand for specific skills. CareerBuilder said high growth in demand for skilled sales representatives, machine operators and nurses was contributing to the number of jobs remaining unfilled long term. Hiring managers frequently said the positions for truck drivers, software developers, engineers, marketing professionals, accountants, mechanics and information-technology administrators, were hard to fill, as well, CareerBuilder said. The pollsters said the results of the survey conducted by Harris Interactive, have a margin of error of plus and minus 2.17 percentage points, and can be said with 95 percent probability.