What does a negative value of Mean Error indicate?

SirajMazumder

The problem is :

"I performed IDW for EMF levels from cell phone towers. The resulting Mean Prediction error in cross-validation is -0.0081."

I've read about Mean Prediction Error and RMS Prediction Error in IDW on ArcGIS websites. But nowhere is it mentioned about what conclusions to draw if the Mean Prediction Error has a negative value

Eric6346

The mean error is the average of all the cross validation errors.  A positive error means that the predicted value is larger than the true value, and a negative error means that the predicted value is less than the true value.  For unbiased models, the underpredictions should cancel out the overpredictions on average, and the mean error should be close to zero.  Sometimes it will be little bit negative, sometimes a little bit positive, but if it is close to zero, you have evidence that the model is unbiased.

Your value literally means that on average, the cross validation predictions were 0.0081 lower than the true values.  Since I can see in your screenshot that your measured values range from about 0.03 to 0.24, a value of -0.0081 is negligible.  If the mean error were instead something like 0.1, that would be very concerning, as the level of bias would be as large as most of the measured values.