The bottom tier of the job market made up in large part of service occupations is resistant to wholesale automation because labor-saving innovation is happening elsewhere in the economy: labor-process refinements in other sectors shed workers who, because they must work to live, are shunted into low-wage, low-skill occupations. This surfeit of available labor creates intense competition for otherwise undesirable jobs, holding wages down. There can be an oversupply of labor in this sector, though, because these jobs are deemed to require little to no pre-existing skill, requiring no special training or aptitude to perform the tasks associated with them. In many cases, workers who hold these positions are trained on the job, or at most subject to short introductory training sessions or programs before they begin.