Tests would tell parents whether their children were doing well enough and how they compared with other students across the country.

This make it difficult to compare performance across different course.

You can't compare the game across time because so many variables have changed.

Using only data that can be compared across countries, the newspaper measures schools from around the world.

Sometimes drugs cannot be compared across a wide range of treatment situations because one drug has a ceiling effect.

Many rhombomeres, if compared across species, do not form into the same things.

The product of the two will give a rough estimate that can be compared across different countries.

It can be especially difficult to compare happiness across cultures.

Number of poor are hard to compare across countries.

Finally, researchers can not agree on the definition of neighborhood, which makes it hard to compare across study.