In Social Explorer, say you zoomed in to Northern Minnesota, then mapped a census variable for $2000($for instance the percent of renter occupied housing units$)$ at the census tract level. Do the same for $1970.$ You would see that the boundaries of census tracts have changed, and that some areas are not even covered by census tracts. What type of problems does this create when one wants to compare the rental situation between $1970$ and $2000?$