By The Associated PressThe U.S. economy added 155,000 jobs in December, slightly lower than November's gain. At the same time, the unemployment rate remained unchanged at 7.8 percent. Why didn't the modest job gain push down the unemployment rate? 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 December, the payroll survey showed that companies added 168,000 jobs, and federal, state and local governments shed 13,000. 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 December, the household survey showed that the number of people who said they were either working or looking for work rose by 192,000. But of that group, only 28,000 said they found jobs. The other 164,000 said they were unemployed. That almost pushed up the unemployment rate, but when rounded off, it remained 7.8 percent. 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.