Date as a Covariate


New Member
Hi, I am looking at the effect of various weather variables on sub-population size of a certain species of snake. Sub-population size at the study site over the active period follows a quadratic shape increasing in the spring to a peak in the summer and drops in the fall. Temperature during that period follows a similar pattern. I want to determine the effect of mean weekly temperature, wind speed, and precipitation on weekly sub-population size. A quadratic regression of sub-population size vs weather would show a significant effect of temperature. However, shouldn't I include a covariate to account for the seasonal variation? The relationship between sub-population size and time is not linear so I have tried using the polynomial 'week^2' as the time covariate but of course this causes the effect of temperature to disappear.

How do I tease apart the effect of temperature and basic seasonal behaviour? Is this possible? Any help on this matter would be greatly appreciated as I have spent many fruitless hours searching in the literature for a suitable solution. Thank you.


TS Contributor
The first question would be: what is temperature nett of season and is its effect the effect you are looking for? I can imagine that the effect of deviations in temperature from season average could sometimes be meaningful, but I doubt whether that is always the case.


Fortran must die
Seasonal variation in what? You should decide what is actually varying (say sunlight or wind or whatever) and specifically model that.
beware of ecological trends
... not "ecological" in the sense that something is a study in the field of Ecology, but trends of two things that happen to occur in a similar seasonality pattern