Weights of fur seals in two years

This diagram again shows that the marginal relationship between Y and X may be misleading if the data are observational.

Marginal relationship between size and time
The overall mean weight decreased from 2003 to 2004
Conditional relationship for each colony
The mean weight in each individual colony is higher — in each colony, the seals appear to be bigger.
 

The reason is that a larger proportion of seals were sampled from colonies with immature seals in 2004.

The marginal relationship was caused by a lurking variable, Colony.


The data are artificial.

The context is that a marine reserve was created in 2002. The data are supposed to be the weights (kg) of seals that were observed along a coastline bordering the reserve in March 2003 and March 2004.