Regression and seasonality

I am running a multiple linear regression analysis to show how different demand factors affect demand of juice. My independent variables are price, temperature, competitor price, feature space and competitor feature space. However, I know there is an element of seasonality which is affecting the demand. I am looking at 2 years worth of weekly data, so feel that including dummy variables isn’t an option. I have seasonality indices which have been loosely worked out using an average of 4 years, removing sales caused by promotion and giving a percentage increase or decrease from how much sales vary on average from the base sale. How can I incorporate seasonality into my regression model?


Less is more. Stay pure. Stay poor.
So for clarification, you do not think your scenario would fall under time series?

Have you plotted the relationship to see if their appears to be a relationship you may need to control for?
I wouldn't use a time series model here as I'm trying to measure to what extent external factors are influencing the demand - there just appears to be a general 'spike' in the demand at certain points during the year


Super Moderator
Maybe a trigonometric (e.g., sine/cosine) function of time would be useful here as a control variable? That'd allow you to control for some types of smoothly shifting seasonal demand.