I have used CCD under RSM to design a set of 13 experiments for 2 factors of 2 levels. In the attached images the results of the responses I got. The response is Max. absorbance and the negative value was replaced by zero. When the absorbance 0 is used, The Predicted R² is not as close to the Adjusted R² as one might normally expect; i.e. the difference is more than 0.2. This may indicate a large block effect or a possible problem with the model and/or data. In addition, the Normal Plot of Residuals is, I think, s-shaped. when the negative results were included in the model. The situation was different: i.e. The Predicted R² was close to the Adjusted R² and the Normal Plot of Residuals looked good. I don't know if it is possible to show the model with negative absorbance, Does it make sense?
Negative absorbance values, particularly those as low as -0.1, suggest potential issues with the experiment, such as instrument calibration problems or the use of inappropriate cuvettes. If it affects all measurments, you might have problems with fitting a model.
