# Difference between: Offset and Weights?

I´d like to know the difference between these parameters when I am using GLM/GLMM/GAMLSS/BETAREG. I have observed a lot of published studies using offset, weights and covariates, however I am not sure about the use of these parameters in models.

Could someone explain to me when should I use each one parameters (offset and weights) and/or if should I use it as covariate?

• Weights are strange, and depend on what program you are using, and even what function within the program. Have you tried the documentation for your program - if so, can you be a bit more specific about what you need to know. – Jeremy Miles Nov 16 '16 at 20:54
• An offset is a covariate but with a fixed coefficient, not an estimated one. – mdewey Nov 16 '16 at 22:00

Offset and weights are very different things. Offset is really a covariable included in a model with a fixed coefficient of $1$, which is not estimated. They are mostly used with poisson models to represent exposure, see Should I use an offset for my Poisson GLM? , When to use an offset in a Poisson regression? and search this site, there are many posts. For example of use with logistic regression, see Using offset in binomial model to account for increased numbers of patients