Questions tagged [gam]

Generalized additive model (GAM) is a generalized linear model (GLM) in which the response variable depends on unknown smooth functions of some predictor variables.

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1answer
19 views

Neighborhood matrix in spatio-temporal model

I am attempting to create a neighborhood matrix (poly2nb in spdep) in R to use either for a GAM or an INLA. As I have spatio-...
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28 views

GAM: Modelling rate of change with non-linear interaction

I want to model the rate of change of Y as (ΔY=Yt-Yt-1) as a function of x1, x2. For that I'm using a GAM model with an interaction as following: ...
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1answer
25 views

Scale of time covariate in `corCAR1()` matters?

I'm running a bunch of GAM analyses on time series data (measurements of my own weight). Unlike many examples I see online, my data is not spaced evenly in time. In fact, time points can come minutes ...
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1answer
24 views

What is the difference in between quasi-Poisson regression model and generalized additive models? On what basis do we choose these models? [closed]

I read some papers where some authors used one model to the other, I tried to find the underlying assumption behind the model but can't fully understand it. It is not clearly mentioned. One reason I ...
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1answer
90 views

Representing a GAM with truncated power basis as a mixed model

The truncated power basis looks like this: $$ y = \theta_0 + \theta_1x + \theta_2x^2 + ... + \theta_dx^d + \sum_{k=1}^K \theta_{dk}(x-\kappa_k)_+^d $$ Apparently, you can just separate the ...
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1answer
33 views

Which model to choose for time of day as a dependent variable having a skewed periodic/rhythmic relationship with some response?

I have a data-set with time of day (0 - 24 hours) as a dependent variable together with some continuous response variable which demonstrate what looks like a skewed sine relationship. For ...
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1answer
59 views

Based on the graph & table, what method is used for the analysis that has been started on the table pictured?

Looking at the time-series plot of data (pictured), and looking at the table (pictured), what method and why has been chosen for the analysis that has been started on the table shown? I'm struggling ...
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3answers
116 views

How to test for the best parameters for transformed independent variable in linear model

Let's assume that I have a linear model with $k$ variables: $y = \beta_0 + \beta_1\cdot x_1 + \dots + \beta_k \cdot x_k$. Now, I want to add variable $x_{k+1}$, but, according to domain knowledge, ...
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9 views

Getting the right interaction structure (continous and categorical) in GAM and intepreting p-values

I have looked long and hard to better understand implementing the right interaction structure in GAM and am aware of similar posts out there so I apologise for that, however I'm still unsure sure how ...
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25 views

Time series regression analysis with GAM in a factorial design

I have data from 16 automated sensors that measure a parameter across 4 experimental treatments with 4 replicated experimental units each: How would I go about to robustly test for significant ...
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1answer
25 views

Aggregating sales forecasts from mutually exclusive segments

I am using generalized additive models (GAMs) to forecast sales for 16 mutually exclusive and exhaustive customer segments. There are naturally correlations in these 16 series, including seasonal ...
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1answer
57 views

GAM Interactions : Individual and Combined Interactions are different

I am quite new to GAM, I was trying various interactions in my GAM models, the individuals interactions and combined interactions are not coming up the same. There are three variables which define my ...
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30 views

Why these three models suggest such a great difference on the significance

Does someone could tell me why these three models suggest such a great difference on the significance of factors? And which one is much reasonable? ...
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1answer
28 views

calculating percentiles (quantiles) from GAM predictions in R

I'm working with a bird migration dataset, and exploring some different metrics to quantify changes in migration phenology (timing) over several decades. There are many different approaches to do this,...
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1answer
31 views

Need some eyes on my dataset: can we argue here that there is a random spread around a constant?

I have this huge data set where I am looking to find trends. However, this one quantity (Figure) seems to randomly spread around an almost constant line (with the exception between 1600-2300). I did ...
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46 views

Similar to beta regression for Generalized Additive Models and Generalized Linear Models

My thesis is based on GAM and GLM. But my dependent variable is a proportion, meaning that $y \in (0,1)$ then I thought a better fit for this was the beta regression but this one is not considered in ...
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1answer
36 views

For non-parametric regression which one has better interpretation and properties, GAM or quantile regression?

As in the topic. I want to interpret data for which I have no clues about the distribution. It's neither count, percentage, continuous. I don't want any transformations. Instead I would like to ...
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22 views

Representation of cubic-linear spline in mgcv::gam

I have a dataset of empirical hazard rates which I would like to model to produce a reasonable forecast of the survival rate. I'd like to use a cubic-linear spline to fit the logit-transformed hazards ...
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19 views

Computational time of a (fairly complex) GAM with ARMA structure in brms

I am fitting a model for time-series analysis of Wikipedia views with STAN through the brms package. I came up with a pretty good distributional model, which ...
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12 views

ordered variables in GLM vs GAM

I'm trying to find the best model from a dataset which mainly has ordinal variables (in likert scale). So, I don't know since I had to put in GLM as.factor do I ...
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1answer
105 views

When and why would you not want to use a GAM?

I understand that GAMs in mgcv have the ability to reduce s(x) to a linear relationship with the response variable. If this is the case, why wouldn't you use a GAM? ...
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1answer
12 views

anova.gam, significant test and 0 deviance

I have fitted a series of GAMs of increasing complexity, and compared with anova.gam. I get a significant p value (based on Chisq test) for a pair of models, even though the difference in deviance ...
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1answer
40 views

Acceptable/reasonable deviance explained for fitted GAM?

thanks to all for any help in advance. I have built a series of nested GAMs in mgcv to explain the presence/absence of antibodies in a population of animals and used AIC to select my best fitting ...
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1answer
39 views

Autocorrelated residuals in GAM even with lagged variables or AR process

I'm using GAMs to model ozone as a predictor, and using time , temperature and another pollutant (poll) as covariates. I have 20 years hourly data (large dataset), but I'm using 2 periods of 10 years, ...
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1answer
23 views

ANOVA for natural splines

I want to use ANOVA for selecting the best model with two independent variables, where either one of them (or both) can be fitted with natural splines. Hence, I try the following models: ...
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26 views

Unusual residual artefacts in GLMM, is GAM or another model more appropriate?

I'm having trouble finding an appropriate model for my data. The data comprises behavioural observations of chimpanzees, where I instantaneously sampled their locomotor behaviours and parameters of ...
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1answer
53 views

Interpreting a pattern in a residual plot produced by gam.check()

I'm working on creating a model that examines the effect of ocean characteristics on fishing outcomes. I have spatial data on a 0.5 degree grid and I created the following model: ...
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1answer
361 views

Smooth bivariate interaction decomposition in GAM models

Background Consider the following additive model containing a smooth bivariate interaction term: $y = f(x,z) + \epsilon$ where y is a continuous outcome variable, x and z are continuous ...
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97 views

How does random variable nesting in GAMs work (mgcv)?

Consider me very new to the world of GAMs, I am actually using it out of necessity rather than by choice but I thought it could be a chance to learn something new anyway. Originally I had hoped to ...
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39 views

How does weights argument in gam() to handle the heterogeneous variance issues

In my case there were multiple observations per Group(random effect) in a single Year. So I aggregate these cases by calculating ...
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1answer
48 views

Make Nonlinear Smooth Interpretable in Logistic GAM Regression

I have a nonlinear smooth fit in a logistic regression from the package mgcv in R. Visualizing the smooth, the y-axis I get using either ...
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0answers
36 views

How to formulate nested random effects in gam

I'm interested in how growth of a fish species is related to several environmental variables. Those variables are measured at the time of capture, but several fish can be captured in a single sample (...
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12 views

Can we get the specific formulation form of thin plate regression spline in GAM?

In mgcv package, the default smoother is thin plate regression spline. After building a GAM model, can we "write" the formula form of the thin plate regression ...
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14 views

Understanding how to weight variables

I am trying to model the "recruitment" of a deer population where recruitment is a factor of "females counted in fall" and "fawns counted in fall". e.g femaleFall:fawnsFall = Recruitment. In some ...
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28 views

Conditional Independence in generalized additive models

I'm grappling with a question that I think I know the answer to, but not able to prove it mathematically. I have two oils, which are each run once on an engine across ~800 seconds while the engine ...
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13 views

What are the alternatives to basis functions like a B-spline or polynomial?

Let $f_1, ..., f_n$ be basis functions so that we consider $F = \sum f_i \alpha_i$ where $\alpha_i \in R$ are constants. This is what we do when we use e.g. generalized additive models. I am ...
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0answers
18 views

Investigating trends (is joinpoint good?)

In investigating trends in surveillance (especially cancer). Joinpoint (https://surveillance.cancer.gov/joinpoint/) is often used. Joinpoint simply estimates a segmented regression with cutpoints (...
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1answer
102 views

How interaction terms are treated in Gradient Boosting?

In GAMs interaction terms have to be expressly specified as covariates, even for simple linear relationships. On the contrary, with Gradient boosting this is not nesessary because the algorithm itself ...
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21 views

Linear combine non-linear transformations

Could someone tell me what it is called if you linear combine a non-linear transformation such as: $$y_i = \beta_1 f(x_{1i}) + \beta_2 f(x_{2i}) + \ldots + \beta_n f(x_{ni}),$$ where $f(\cdot)$ is a ...
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0answers
51 views

Interpolating time ordered smooth function into space

I am using GAMs in R to generate daily weather variables, mainly precipitation and temperature. Currently, I am fitting a model for each weather station but I would like to use it in places where ...
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1answer
34 views

Marginal effects of a smooth in a gamm4 model

I'm trying to obtain marginal effects of a smooth in a {gamm4} model. I notice a discrepancy between what {ggeffects} gives me and what I get manually. For a smooth x0, I calcualte the predictions ...
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1answer
78 views

rules determining nesting in generalized additive models

I understand the concept of nesting when all of the free parameters are clearly identified in linear or generalized linear models (one model is nested within another if its free parameters are a ...
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0answers
60 views

GAM model: Group-specific smoothers with different wiggliness of two random and nested factors [closed]

I aim to model the specific seasonal population fluctuations of several species. In particular, I have the abundance of individual along several years of 20 populations belonging to 5 species, and I ...
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1answer
355 views

visreg visualization of mgcv results (GAM)

I fitted a GAM with random effects using mgcv, and I've noticed that the visualization of the smooths using visreg does not appear to match the output of mgcv's plots: ...
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40 views

First derivative of splines in bayesian model

Inspired by this post https://www.fromthebottomoftheheap.net/2014/05/15/identifying-periods-of-change-with-gams/, I'm trying to identify periods of change in a GAM model, using bayesian inference. ...
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1answer
92 views

Why is there “residual dots” in plot() of GAM?

After building a generalized additive model (GAM) using mgcv package, we can use the plot function to visualize the smoother, ...
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0answers
15 views

Regression with a growing year over year dataset

I created a hierarchical GAM to model final event sales as a function of sales to date, days until event, teams that are playing, and event month with a grouping at the home team level. This yielded ...
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2answers
60 views

For a mixed model, does the temporal correlation should be checked for each site (random effect)?

When a model fitted in a dataset with multiple time series from different sites, does the temporal correlation should check for each site? And if some sites show auto-correlation but some don't, what'...
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1answer
51 views

How to fit a zero-inflated quasi-poisson distribution to a continuous variable using GAM [closed]

I am trying to fit a GAM to a continuous variable which is zero-inflated. However, since my variable is continuous, I am not able to use ziP() for a zero-inflated quasi-Poisson. Is there someway to ...
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0answers
24 views

Regression with sparse data - binning or other approaches?

A couple years ago, I built a GAM to predict game sales regressing away team, sales to date, event month, and days to event against final event sales using smoothing parameters over days to event and ...