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Questions tagged [offset]

A variable with a fixed coefficient of 1. Offsets are commonly used in Poisson regression to allow the modeling of rates instead of counts.

9
votes
3answers
1k views

Modelling count data where offset variable is 0 for some observations

I'm trying to help a student of a colleague. The student observed and counted bird behaviour (number of calls) in an experimental setup. The number of calls attributable to a specific observed bird ...
6
votes
1answer
996 views

When to use offset() in negative binomial/poisson GLMs in R

I'm trying to detect relationships between species abundances (counts) and time (years) for many species using either Negative Binomial or Poisson regressions (depending on degree of dispersion). ...
12
votes
2answers
6k views

Binary Models (Probit and Logit) with a Logarithmic Offset

Does anyone have a derivation of how an offset works in binary models like probit and logit? In my problem, the follow-up window can vary in length. Suppose patients get a prophylactic shot as ...
2
votes
1answer
1k views

glm with offset for rate data

I am attempting to fit a GLM to rate data. In my case it is the number of provisioning visits to a bird nest per hour. I model the data like in the example below including time as an offset. ...
2
votes
0answers
833 views

How to estimate an mlogit (R package) model with a fixed (offset) variable

I am trying to estimate a multinomial logit model using an offset variable with the mlogit package of R Using the syntax here [https://stat.ethz.ch/R-manual/R-devel/library/stats/html/formula.html] ...
1
vote
0answers
152 views

Regression Model - forcing a coefficient/effect to be 1

Currently we have a linear model which includes 3 independent variables and the dependent variable "Y" which is the predicted values (range from -0.5 to 2000) from the model. Now we want to leverage ...
4
votes
1answer
510 views

Offset in Poisson regression

Consider an offset term in a Poisson regression: $$\log \mu_x = \log t_x+ \beta_0 + \beta_{1} x$$ To interpret $\beta_0$, would you need to consider $(\beta_0+ \log t_x)$? Because what if $t_x \neq ...
1
vote
1answer
463 views

Offset variable in Poisson regression makes model worse

I'm using a Poisson regression to model count data (number of orders). My observation lengths are different, so I tried to include an offset variable in the model. The problem is that when I estimate ...
1
vote
2answers
653 views

Assessing performance of linear model with fixed slope

I have a very simple linear model that I need to fit with a fixed slope (I'm interested in estimating the intercept). How would I go about describing the performance of the fit, since the usual r^2 ...
1
vote
0answers
831 views

Analysis of rates and post hoc test with offset variable using glmer.nb

I am investigating variation in pollinator visitation rate (number of visits per inflorescence) with treatment and time category as fixed factors. Block is a random factor. Following Zuur et al. (2009)...
1
vote
2answers
557 views

Negative Offset in Rate (Poisson or Negative Binomial) models

I have a dataset that contains: the counts of successes, $Y_i$ the length of observation, $\text{length}_i$ few predictors, $X_1, X_2, \text{etc}$ Since the counts are observed at different lengths ...
4
votes
1answer
5k views

Calculating predictions and confidence intervals from a negative binomial distribution

I'm trying to use R's glm.nb to calculate predictions and confidence intervals. When I'm using linear models after training a model, e.g., using: ...
22
votes
2answers
27k views

Where does the offset go in Poisson/negative binomial regression? [duplicate]

(First of all, just to confirm, an offset variable functions basically the same way in Poisson and negative binomial regression, right?) Reading about the use of an offset variable, it seems to me ...
4
votes
1answer
5k views

Predicting with GAM, using an offset

I came in a new post because I’m quite confused with the use of an offset in gam() function (mgcv package). I’m actually working on predicting marine top predator’s distribution using aerial surveys (...
5
votes
0answers
1k views

Analysis of survival data using binomial GLM with offset

We are interested in determining whether there's an association between frequency of screening visits and cancer outcomes and whether that differs by race. We have Medicare data to analyze this. ...
17
votes
4answers
19k views

Using offset in binomial model to account for increased numbers of patients

Two related questions from me. I have a data frame which contains numbers of patients in one column (range 10 - 17 patients) and 0s and 1s showing whether an incident happened that day. I'm using a ...