[R] GLM and WLS

[frespider]

Hi 

I am running some simulation in  R after resampling from a huge data set 500
columns and 86759 rows and fit these two model and calculate R-square 
fit <- lm(Y~X,weights = Weights)
bhat <- coef(fit)
Yhat <- X%*%bhat                         
SST <- sum((Y - mean(Y))^2)              
SSE <- sum((Y - Yhat)^2)               
MSE <- SSE/(n-pc)                  
aRsqr <- 1-(((n-1)*SSE)/((n-pc)*SST)) 

and the GLM Model

fitGLM <- glm(Y~X,family=Gamma(link = "log"))

Yhat <-exp( X%*%bhat )
SST <- sum((Y - mean(Y))^2)              
SSE <- sum((Y - Yhat)^2)               
MSE <- SSE/(n-pc)                  
aRsqr <- 1-(((n-1)*SSE)/((n-pc)*SST)) 

I am getting a negative R-square which I don't understand why? Can some one
let me know why and if I can investigate that how I do that.

Thanks



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