# Correlation between two time series of sales data

I have two time-series of 2 different products x & y. They belong to the same main category, e.g.: Iphone 6 and Iphone 5. Maybe one product is the predecessor of the other product.

I tested for correlation between the sales of the 2 products and got r=.67. Correlation is calculated on the number of sales of the 2 products for the same month. Does this mean that there is a causal relationsship between the sales of the two products? Are both sales pattern the same? In this case I might be able to forecast one time series with the historical data of the other one.

• Probably not, correlation does not imply causation most of the time. The reason for stock X and stock Y rise together might not be that rise of X cause rise of Y or the other way around. It might be that rise of Z causes the rise of both X and Y. Rise of X might be a good indicator (in a short time) that Y is going to rise soon. But that doesn't mean X causes Y. – hamster on wheels Aug 4 '17 at 21:43
• By the way, why it takes a year for cross-validate to have an answer for this ... – hamster on wheels Aug 4 '17 at 21:44