In linear systems, effect is 'always' directly proportional to cause.

"What I believe is that it's directly proportional to hard work and experience," he said.

The quality of the image produced is directly proportional to the frequency used.

"It's directly proportional to the way we do on the field," Williams said.

Attention to weight seems to be directly proportional to how far you are hiking.

It is directly proportional to the mass of air over that location.

The result of the test/exam will be directly proportional to the daughter's status.

To these the extent of death is directly proportional to their personal welfare.

Job happiness is directly proportional to the distance you are from the home office.

It might have been thought that the time would be directly proportional to the distance.