Here are five reasons that might explain the disparity:
1. You co-worker negotiated better than you did when she was hired.
There’s a lot of variation in whether and how people negotiate salary when they get a job offer. Some people accept on the spot, others push for a little more, and others push for a lot more. And some of them get it. Your co-worker’s salary might be higher than yours simply because she asked for more. (Unfortunately, asking for more now probably won’t close the gap; it’s never as easy to negotiate after you’ve accepted the job.)
2. The job market was tighter when your co-worker was hired.
In job markets like this one, employers can hire good people for lower salaries. But a few years back, when jobs were more plentiful, employers had to offer more money to attract the best people. If your co-worker was hired during an employee’s market, and you were hired during an employer’s market, that could explain the difference.
3. Your co-worker has a particular degree or skill that the company rewards.
Even if you and your co-worker are doing roughly equivalent work, the company may put people with certain skills, degrees, or certifications into a higher salary category.
4. Your work isn’t as good as you think it is.
A lot of people overestimate their own performance, and they often have trouble recognizing that’s the case. It’s worth considering whether there might be good reasons why the company might not value your work as highly as they value someone else’s—even if it’s a blow to your ego.
5. Your co-worker’s boss or job is a nightmare.
If your co-worker’s job is particularly difficult or unpleasant, the company may pay more to attract and retain people willing to do the work, even if the skills involved are roughly the same as yours. The same could apply if her boss is the problem; it’s not unheard of for a company to increase the salaries of people working under a jerk to keep employees from leaving.
So what can you do about it?
Despite each of these scenarios, you might be able to improve your own salary if you’re dissatisfied. But you’ll have a better chance of reaching your pay goals if you focus on the salary you deserve, completely independent of what your co-worker makes.
Do some research on industry norms for your particular work in your geographic area and see where your salary falls relative to those markers. If your research shows that your pay is roughly in line with what makes sense for your industry and the only issue is that your co-worker makes more than you, you can still ask for more, but be open to the idea that the pay raise might not happen. That’s not an insult, just a pretty typical result of the way different people negotiate different packages.
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You can also ask your boss what you would need to accomplish to earn a raise. That conversation could lead to valuable feedback, which might help you discover a solid path to the salary you want.
Ultimately, if you don't you don't like your salary and your boss won't budge, go out there and see what other offers the world has for you. You might find one you like better—or decide you'd rather stay put. Just make sure you base that decision on what the job is worth to you, not to your colleague.
Alison Green writes the popular Ask a Manager blog where she dispenses advice on career, job search, and management issues. She's also the author of Managing to Change the World: The Nonprofit Leader's Guide to Getting Results and former chief of staff of a successful nonprofit organization, where she oversaw day-to-day staff management, hiring, firing, and employee development. She now teaches other managers how to manage for results.