Consider a (simple) Poisson regression . Given a sample where , the goal is to derive a 95% confidence interval for given , where is the prediction. Hence, we want to derive a confidence interval for the prediction, not the potential observation...
Posts Tagged ‘ glm ’
Test Difference between Two Proportions & Plot Confidence Intervals
..an illustrative example for testing proportions and presenting the results.the data: number of indigenous and alien plant species with and without vegetative reproduction. Hypothesis: The proportion of species with vegetative reproduction is differen...
Model Validation: Interpreting Residual Plots
When conducting any statistical analysis it is important to evaluate how well the model fits the data and that the data meet the assumptions of the model. There are numerous ways to do this and a variety of statistical tests to evaluate deviations from model assumptions. However, there is little general acceptance of any...
Generalized Linear Models – Poisson Regression
The Generalized Linear Model (GLM) allows us to model responses with distributions other than the Normal distribution, which is one of the assumptions underlying linear regression as used in many cases. When data is counts of events (or items) then a discrete distribution is more appropriate is usually more appropriate than approximating with a...
My residuals look weird… aren’t they ?
Since I got the same question twice, let us look at it quickly.... Some students show me a graph (from a Poisson regression) which looks like that, and they asked "isn't it weird ?", i.e."residuals are null or positive... this is not what we...
Studying joint effects in a regression
We've seen in the previous post (here) how important the *-cartesian product to model joint effected in the regression. Consider the case of two explanatory variates, one continuous (, the age of the driver) and one qualitative (, gasoline ve...
Bookshelf remodelling
I found time and read Gelman and Hill’s “Data Analysis Using Regression and Multilevel / Hierarchical Models“…Now, please do yourself a favour and get it (of course the paperback version ). Even for experienced or intermediate (myself) this will be a treat for your eyes and neurons. PS : (Confession) I didn’t like the...
R Commander – logistic regression
We can use the R Commander GUI to fit logistic regression models with one or more explanatory variables. There are also facilities to plot data and consider model diagnostics. The same series of menus as for linear models are used to fit a logistic regression model. Fast Tube by Casper The “Statistics” menu provides access to...
Measuring the length of time to run a function
When writing R code it is useful to be able to assess the amount of time that a particular function takes to run. We might be interested in measuring the increase in time required by our function as the size of the data increases. To illustrate using the system.time function to calculate the time taken...
Offset in glm ()
To add an offset to the linear predictor of a generalized linear model (or models from the survival package such as coxph and clogit), use offset(x) in the formula. This will add an offset to the linear predictor with known coefficient 1.
Zero Inflated Models and Generalized Linear Mixed Models with R.
Zuur, Saveliev, Ieno (2012).
