I’m not sure what data you’re looking for - I mean it’s great that you’ve been able to negotiate the terms of every job you’ve had since college. I’ve been able to negotiate at none. A couple were union jobs where the terms and conditions were initially negotiated before I got there , and I as an individual had no real influence when the contract expired and a new one was negotiated. But the rest- everything from part-time jobs in high school and college to my current position as a manager at state agency , the terms and conditions were imposed by fiat and my only choice was whether to take the job or not. Lots of jobs are like that- do you imagine that Walmart negotiates with every cashier regarding their pay and vacation time and benefits ? Or that your doctor’s office negotiates those items with every receptionist, biller and medical assistant? For an awful lot of jobs it’s a matter of “These are the terms. Take it or leave it”
The same thing goes for almost any contract I can think of that I was involved in - I didn’t get to negotiate the terms of my cell phone contract, or my internet provider contract ,or my credit card contracts or my mortgage. In some cases, I could choose from different plans with different prices , but I couldn’t “mix and match” - I couldn’t negotiate the price on a one year commitment down to the price on a two year commitment. Sure, I could go to another provider, but that’s not negotiating.