It's also worth noting that sunk cost is often strongly correlated with remaining cost. If you've spent 4 years in a 5 year PhD program and start questioning your reasons for being there, you could frame it in terms of the sunk cost fallacy: "just because I've been here for four years, doesn't mean that I should continue on". On the other hand, there's a rational argument for staying: "at this point I only have one more year to go and if I leave now I leave empty handed but if I just stay one more year, I'll get a PhD".