Hello everyone
I am analyzing data from an experiment with 10 treatments, 4 blocks repeated in 13 moments.
One of the variables has zero values until the fifth or sixth moment. After that, the measurements start to appear. I thought about adjusting some type of model considering a threshold parameter, perhaps with a gamma or exponential distribution, and comparing the treatments at the moments.
I would like to know if it would be possible to adjust it like this or, better yet, a model with repeated measurements? How would you consider the beginning of the data? Like to hear suggestions.
Thank you in advance
You didn't provide any information about the nature of your response variable, but it sounds like it might be positively- or at least nonnegatively-valued. Such responses could be modeled using a generalized linear model using various distributions. If it is a discrete count response, then you could use the Poisson distribution (or negative binomial if it exhibits extra variability or excessive zeros). If it is continuous and positive, then the gamma, inverse Gaussian, or Tweedie distributions are commonly used. With these distributions, it is common to use the log link function to model the log of the distribution mean.
But yes, if your subjects or items are repeatedly measured, then the implied correlation should be taken into account by using an appropriate model such as a Generalized Estimating Equations model (PROC GEE) or a random effects model (PROC GLIMMIX) depending on the purpose of the model. Both procedures can use distributions like Poisson and gamma. See the examples in the documentation of those procedures. Also see this note on modeling positive, continuous responses and this note on modeling count responses using various distributions (though this is primarily about handling overdispersion).
ANOVA, or Analysis Of Variance, is used to compare the averages or means of two or more populations to better understand how they differ. Watch this tutorial for more.
Find more tutorials on the SAS Users YouTube channel.