Problem
A researcher recruits 100 people over age 65 living in a retirement community to take part in an observational study. She surveys them about the number of puzzles they do per week and evaluates them with a cognitive functioning test. The researcher finds a significant positive relationship between the number of puzzles and scores on the cognitive function test. She publishes her results in an academic journal. She writes in her conclusions "Doing puzzles caused an improvement in cognitive function in this study." Which of the six tests of science has the scientist violated with this last statement?