The Math Behind Surrendering
Assuming: Avg LP Gained Per Win = 20 LP Avg LP Lost Per Loss = 20 LP Avg Game Length 30 Minutes
The expected value per minute of surrendering at 20 minutes is -1 LP / Min.
According to www.leagueofgraphs.com, the probability of winning after losing key objectives is as follows:
First Inhibitor 8% First Baron 18% First Dragon 31% First Tower 27% First Blood 40%
Therefore, if you play out a game you were expecting to lose at 20 minutes to 30 minutes, the average game length, the expected value per minute of the game becomes:
First Inhibitor -0.56 LP / Min First Baron -0.43 LP / Min First Dragon -0.25 LP / Min First Tower -0.30 LP / Min First Blood -0.13 LP / Min
In other words, even in the worst statistic, losing your Inhibitor first, and having only an 8% likelihood of winning, not surrendering improves the value of your LP / Min by 44%.
Remember this the next time you hit that surrender button at 20 minutes.
Edit: Updated values - They were even better than I thought. Thank you to No to Toxic for catching my math error. :P