What metrics do you use to justify a pay raise?

I think this blog post https://www.thebalancecareers.com/how-to-succeed-at-asking-for-a-pay-raise-2071868 does a nice job walking through key considerations and things to do when generally asking for a raise, including specifically in the “Research” section highlighting some of the classic sources of data to use for salary comparisons and other elements of total compensation. The article does also give some tips around having the conversation and on those points, I just wanted to add that I definitely think the ask should happen in a meeting, not over email, and I don’t think it’s usually necessary to ask for a specific meeting to discuss a raise (which makes it seem very formal) when you could just add it as a topic in your next 1:1.

For metrics specifically to justify your raise, I would say use anything that shows you’re outperforming targets or even your peers or other areas of the business (although you of course don’t want to be putting others down). This could be beating the goals set for a major launch, delivering well above metrics like pipeline and bookings for your areas, getting very high satisfaction scores on your presentations, etc.

Asking for a raise is tricky. Ultimately, you need to be driving value, right? That can be broken down quantitatively, but also qualitatively.

Quant: What impact are you having on funnels? Run A/B tests to prove that your strategies are driving impact. How have NPS and sentiment changed?

Qual: Do you have strong relationships with stakeholders? Are you driving value through strategy, creative, and channel partnerships?

I would also recommend using your companies job ladder as a tool, or if you don't have one, job descriptions for other similar roles. If you're a PMM and the expectations for a PMM are X. But you're operating at Y level, which are the epxectations for a Director of PMM, then you should surface that. If you're exceeding expectations for your role, make that known during your review.

Comps may be a useful tool as well. Look at what other companies are paying for your role if you think underpaid. You need to have some credible target to aim for.

The previous answers here are a bit too pessimistic, I think. First and foremost, benchmark your comp against peers at LinkedIn and Glassdoor (Follow-up question for the community: What other salary aggregators do you use?) This can prove your compensation is too low, if it is.

Beyond that, use objective and subjective proof of your value. Objective: Your impact on pipeline and aspects of sales funnel that you touch. This is expecially critical for B2B. Subjective: Strong reviews from your company's best sales people and commercial executives. Bring this proof to your boss and ask them to advocate for your raise.

Keep in mind that if you're not perceived as a strong performer, nothing you do or say is likely to get you a bump beyond COLA/corporate performance bonus.

Also be prepared to be patient. Find out in advance what the pay adjustment cycle is at your companies. Most will only consider raises once or twice a year. Be sure you are making your case well in advance of the cycle--60 days is good.

The brassiest option? Bring your boss a competing job offer from another company at a higher salary. And be prepared to take that offer if your company says no...

To be honest, I wouldn't waste my time asking for a raise. You generally only get incremental bumps while staying in the same position. You can sometimes expect a moderate raise when you get promoted, but generally speaking if you want a big raise you're going to have to go to another company. Do this while the job market is good - it's not always an option (see 2000-2003, 2008-2010). 

In my opinion, this is the wrong question to be asking. A much better objective would be to aim for a promotion, not a pay raise. Of course, promotions carry pay raises, but the point is that a promotion is more easily understood and communicated. It's easier to ask your boss "what do you think I need to demonstrate to earn a promotion to [title]" than to ask your boss "what do I need to do to get a 15k raise". 

The above answer assumes that you are being paid roughly market rates. If you are junior and inexperienced, don't expect to pull the median salary. By definition, half of people make less than that number, and you're rightly in that group. However, if you have some experience and can show a solid track record of delivering for the company, by all means have a conversation about salary adjustment to bring you to median or some amount over that seems right. That's a dispassionate conversation similar to readjusting a product's revenue forecast given the past 6 months of sales. Be direct and ask for what you want. Don't play games by going and getting a counter-offer.