Tagged Questions

0
votes
2answers
48 views

Transforming nonlinearity in genearlized linear models

Are the buldging rules applicable in generalized linear models? Specifically, to transform the independent variables? I've only seen it disscused/used in OLS regression. Thanks in advance
3
votes
1answer
286 views

GLM vs square root data transformation

I am currently analysing some pretty awful/awkward data on the abundance of fish under three different "Hydro-Regimes" (5 abundance measurements for each regime - Short/Medium/Long). The current ...
18
votes
1answer
655 views

Why is the square root transformation recommended for count data?

It is often recommended to take the square root when you have count data. (For some examples on CV, see @HarveyMotulsky's answer here, or @whuber's answer here.) On the other hand, when fitting a ...
0
votes
0answers
91 views

GLM quasi family with log transformation?

I am working on my thesis analysis, and I have some error data that's right-skewed. I log-transformed it and ran glm on it (gaussian, identity in R) weighted by sample size, and my data is still ...
3
votes
3answers
259 views

Can I put Log(Y) as a dependent variable in a count data model

I have count data passenger as Y. The data look like this, as many of the values are 1 (about 18%.) Does it make sense that I take a log of it, and take it as a dependent variable in a generalized ...
1
vote
0answers
308 views

Interpretation of a log likelihood function for PROC NLMIXED in SAS

I have a data set of skewed nutrient intake values, from around 7800 individuals, of whom around 3000 had two measures of daily nutrient intake (the others only had one measure), so this is a repeated ...
2
votes
1answer
302 views

Running transformed data with a Generalized Linear Model in SPSS

I am running a generalized linear model in SPSS on data highly skewed to the right with a bunch of zeroes (average # caterpillars per tree species branch sampled) so I decided to use the Tweedie ...
3
votes
2answers
358 views

Are ecologists the only ones who didn't know that the arcsine is asinine?

Proportion, ratio, and percentage data is very common in ecology (eg, % of flowers pollinated, male:female sex ratio, % mortality in response to a treatment, % of leaf eaten by an herbivore). An ...
5
votes
2answers
877 views

Transformation to fit gamma distribution for glm

The data simulated below has a maximum value of 4 and is interestingly skewed. The maximum of 4 is a limitation imposed by the instrument used and the data is semi-discrete, i.e., there are a ...
0
votes
1answer
95 views

What assumptions do I need to check to combine levels in a categorical predictor for use in a GLM?

If I have a GLM with a number of explanatory variables, where one is a categorical variable with levels "no treatment" "treatment A" "treatment B" Assuming that it is reasonable to combine the 2 ...
11
votes
2answers
4k views

Transforming proportion data: when arcsin square root is not enough

Is there a (stronger?) alternative to the arcsin square root transformation for percentage/proportion data? In the data set I'm working on at the moment, marked heteroscedasticity remains after I ...