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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
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  • $\begingroup$ This is hard to follow. Can you provide a small example dataset to illustrate your situation? $\endgroup$ Jul 28, 2017 at 14:18
  • $\begingroup$ I have edited this for English grammar based on my interpretation of what you are saying. Please ensure it still says what you want it to. $\endgroup$ Jul 28, 2017 at 14:21
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    $\begingroup$ Note that it does not make sense to compare a GLMM & a GEE model, as they are answering different questions. It may help to read this. $\endgroup$ Jul 28, 2017 at 14:23

1 Answer 1

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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?

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