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I am analysing a data of number of aphids with variation over time (32 avaliations performed every week), area (two areas), and plant position (fixed factor), with 3 replications. Because of that, I am running both GEE and GLMM (to compare the results). The problem is that for each time in each area I have the data on precipitation and temperature. And these data are important because the aphid population is highly affected by these ambient conditions. I really want to study the effect of temperature and precipitation, and I am thinking of using them as fixed factors, but it does not sound correct because they are a range of values. I have thought about using the values as an offset in the model, but I don't know if it is right and probably I will not be able to make any conclusions on how the temperature and precipitation affected the data.

Here is a small example of the data set:

  • REP- repetition
  • posi - plant position
  • prec - precipitation
  • medt maxt mint - temperature information
  • area- area
  • cont- number of aphids
  • time- week of the count
rep   posi prec   medt    maxt    mint   area cont time

1 s   11.03   21.60   28.20   15.10   1   22     1


1 m   11.03   21.60   28.20   15.10   1   0      1


1 i   11.03   21.60   28.20   15.10   1   0      1


2 s   11.03   21.60   28.20   15.10   1   2      1


2 m   11.03   21.60   28.20   15.10   1   0      1


2 i   11.03   21.60   28.20   15.10   1   0      1


3 s   11.03   21.60   28.20   15.10   1   1      1
.   .     .       .       .        .     .   .


.   .     .       .       .        .     .   .


1 s   143.70  22.66   26.24   19.09   2   0      32


1 m   143.70  22.66   26.24   19.09   2   23     32


1 i   143.70  22.66   26.24   19.09   2   18     32


2 s   143.70  22.66   26.24   19.09   2   0      32


2 m   143.70  22.66   26.24   19.09   2   0      32


2 i   143.70  22.66   26.24   19.09   2   0      32


3 s   143.70  22.66   26.24   19.09   2   3      32


3 m   143.70  22.66   26.24   19.09   2   0      32


3 i   143.70  22.66   26.24   19.09   2   3      32
kjetil b halvorsen
  • 77,844
Fabio Janoni
  • 165

1 Answers1

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More of a too long comment.

I really want to study the effect of temperature and precipitation, and I am thinking of using them as fixed factors, but it does not sound correct because they are a range of values. I have thought about using the values as an offset in the model, ...

Offsets are mostly used for modeling rates, and temperature/precipitation are not involved in rates, so modeling them using offsets does not seem right. Apart from this, that temperature information is max/min-values, is a separate problem, but it is not solved by offsets! Maybe use max/min-temp as separete predictors?

kjetil b halvorsen
  • 77,844