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As you move from 0 out to around 1,000,000 or so notice how, within a given interval, the pattern of residuals as you move along either has a number of clusters mostly above the 0 line or below the 0 line.Ĥ. What happens if you log the Y value and re-run the analysis?ģ. The residual plot gives the sense of trending toward a funnel shape and given the size of the fitted values this isn’t too surprising. The three extreme points to the right on the residual plot could be the tail wagging the dog – what does the analysis look like with those removed?Ģ. The normal plot, the residuals against predicted and the time order all appear to have some issues.ġ. The summary stats look great – the plots don’t. You should be able to get these books through inter-library loan. Introduction to Linear Regression Analysis – Montgomery In order to understand what your graphs are telling you you will need to get a good book on regression analysis and read the chapters on residual analysis.Ī couple of references – Regression Analysis – Draper and Smith then you will need to go back and investigate. If you have trends/shapes/clustering/influential data points/etc. Pay particular attention to the way your replicate responses plot over time. Next you should look at the residuals vs the levels of the independent variables and ask the same questions.įinally, you should look at the residuals in time order of experimental run to see if there are any trends. If all is well, and given that you have a significant lack of fit it probably isn’t, then your residual plot should look like a shotgun blast with no obvious trending/clustering/patterns. You should first look at a plot of residuals against predicted to see if there are any important trends/shapes. The first order of business is plot the data. What you, as an investigator, need to do is run regression analysis and that requires far more than just looking at some summary statistics on a printout. In short, you, or rather the machine, took some data and ran a regression. I’m not trying to be mean spirited or snarky but you appear to have done nothing more than build a central composite design, dump the results into a program and hit run.