Root mean squared error (RMSE) is the square root of the mean squared difference between predicted and actual values. It measures how far predicted ratings miss actual ratings; lower values indicate better model fit. section 9.3.2
Use RMSE to evaluate recommendation models by:
- Comparing RMSE across training passes rather than treating its absolute value as universally meaningful, because the rating scale affects the calculation. section 9.3.2
ALS recommendation evaluation
An ALS model configured with maxIter=3, rank=10, regParam=0.15, and implicitPrefs=True produces an RMSE of 1.0007877733299877 on held-out data. Increasing iterations or tuning rank may reduce RMSE. section 9.3.2






