What is a example of a prinapal in science

An example would be to tell someone the truth and to be straight forward with a diagnostic someone may have. 

Here's one prinapal:

Controlled experiments (deliberate manipulation) allow us to infer causal effects.  With observational data, it's never totally clear whether X causes Y or whether X is merely correlated with Y.  

This is why Galileo dropped balls of different masses (but the same size and shape) to see how fast they would fall.  It's a deliberate manipulation of mass, while holding size and shape constant.  Without the experiment, Aristotle and others were inclined to say that heavier objects fall faster than lighter objects do.  With the experiment, Galileo could see that they fall at the same rate.

Now we use the same controlled-experiment principle in the biological and social sciences.  For example, we see that most basketball players are tall.  If we're not careful, we might be tempted to infer that playing basketball causes one to grow taller.  We can test this hypothesis by randomly assigning some kids (but not others) to get invited to  basketball camp.  If we draw from the same population and get two groups who are otherwise identical except for the treatment, we can learn for sure whether playing basketball makes you taller. 

Similar experiments can tell us how much raising the wage increases employee effort, how much taking statins reduces the risk of heart attack among those with high cholesterol, how much raising the tax on cigarettes reduces the number of cigarettes smoked, or by how much digital advertising increases the probability that someone makes a purchase.  (The latter  is an example of something I've done research on recently.)