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.
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.