How can reseachers prevent mistake ans errors in thier experiments

People who exercise for an hour may have lower cholesterol levels. is that a statement? a law? or a hypothesis?

What was the purpose of mendel's experiments with dihybrid crosses? a. to determine if dna was a transforming factor b. to determine if traits could be recessive c. to determine if traits affected each other d. to determine if traits had more than one allele

This is collection of data made by comparing objects in standard units. in science, the units are metric.