https://github.com/gSulpizio/isotherm-fitting/blob/103942c94a6a7b303eb73c062948a724a3221266/src/model%20selection/simpleMSE.ts#L7 it is the sum of squared errors in your case
isotherm-fitting/src/model selection/simpleMSE.ts
Line 7 in 103942c
it is the sum of squared errors in your case