By The Associated PressThe U.S. economy added 157,000 jobs in January, a modest gain, and hiring was much stronger at the end of last year than previously thought. But the unemployment rate ticked up to 7.9 percent from 7.8 percent. How can the unemployment rate rise when the economy also added jobs? Because the government does one survey to learn how many jobs were created and another survey to determine the unemployment rate. Those surveys can sometimes produce different results. One is called the payroll survey. It asks mostly large companies and government agencies how many people they employed during the month. This survey produces the number of jobs gained or lost. In January, the payroll survey showed that companies added 166,000 jobs, and federal, state and local governments shed 9,000. The other is the household survey. Government workers ask whether the adults in a household have a job. Those who don't are asked whether they're looking for one. If they are, they're considered unemployed. If they aren't, they're not considered part of the work force and aren't counted as unemployed. The household survey produces each month's unemployment rate. In January, the household survey showed that the number of people who were unemployed and looking for a work rose 117,000 to 12.3 million, while the size of the labor force did not change. That was enough to push up the unemployment rate. Unlike the payroll survey, the household survey captures farm workers, the self-employed and people who work for new companies. It also does a better job of capturing hiring by small businesses. But the household survey is more volatile from month to month. The Labor Department surveys just 60,000 households, a small fraction of the more than 100 million U.S. households. By contrast, the payroll survey seeks information from 140,000 companies and government agencies â¿¿ and they employ roughly one-third of non-farm employees. The employers send forms to the Labor Department or fill out online surveys noting how many people they employ. They also provide wages, hours worked and other details.