system
Given the following post, respond as if you're commenting on Reddit:
human
How would you measure the Marginal product of labor for someone like a janitor?

We learn that wages are paid at the rate of marginal product of labor. However, since janitors don’t really produce anything or contribute to production for a firm, how would you even measure that or determine their wage? Would you simply need to use the market wage instead?
gpt
Presumably having a janitor generates value for the firm, or the rational firm would not hire them. So ask yourself why a firm would hire a janitor. They keep the place clean, which almost certainly improves labor productivity of other workers. People are less likely to get sick or injured in a clean and well-maintained workplace. Janitors will also perform certain repairs, which also improves labor productivity, and potentially the productivity of capital as well. A clean workplace will also attract better employees who are not willing to work in a dirty environment.

So a janitor definitely generates value for a firm, even if they're not 'producing' anything. In theory, estimating how much more productive labor is because you have a janitor is difficult to do. But a market wage would be a good estimate.