# Interpreting a residuals vs fitted plot and extracting points

I'm doing a multivariate linear regression with R, and i find myself with the following residuals vs fitted plot: As you can see there is a very regular line of points that seems to follow a precise pattern.

My questions are:

1. How do I interpret such a behavior, and what can I do to fix it?
2. Is there a way to isolate/extract those points? I'd like to take a look at them individually in my data set to see if by examining them I notice some patterns in the data.

v.lm = lm(sqrt(v.stima\$Y)~., data=v.stima)


Y is a count-variable (non-negative integer). I'm using sqrt because without it the plot has the typical "funnel" shape that indicates a non-homoscedastic error.

• More information would help. My first guess: those are y=0 values. Is y a count-variable? – Glen_b -Reinstate Monica Apr 29 '13 at 8:52
• Edited with additional info. – Master_T Apr 29 '13 at 9:05
• If you're asking about the line that goes down diagonally at the left side, that's when y=0, as I suggested before. It's what you often see with count data. Why not use a Poisson glm (or other suitable-for-count-data glm), though? – Glen_b -Reinstate Monica Apr 29 '13 at 10:56
• I will search some tutorial/article about the Poisson glm, thanks for the suggestion. As you can probably guess from my ignorance, this is not my field of expertise, I'm a programmer not a statistician, but I have to to this model (for reasons I won't bore you with) so I'm trying to cobble together some knowledge from a book I have and online sources. If you know of a good tutorial about the Poisson GLM, a link would be really appreciated ;) – Master_T Apr 29 '13 at 15:39
• Hi all, the question got accepted, yet the poster says in his comment that the solution does not work. What should we learn from this? – famargar Mar 10 '17 at 15:42

plot(predict(v.lm), residuals(v.lm))