I could never reproduce it well in the lab, because it's really not true. Take a heavy cube the shape of a book. Orient it so that the spine is on the floor. It's a lot more friction to move it in one direction than in the transverse direction. Yet the normal force is the same. Any kid knows this, and I feel dumb it never occurred to me till someone pointed it out to me.
What you are describing (if the normal force is actually the same) is a contact situation where the coefficient of friction is different in different directions (anisotropic friction.)
We can do this with any pair of values, such as current and voltage, but it's useful when the graph between current and voltage is close enough to linear, which means the corresponding coefficient is approximately constant. Well, is it? You have to show that with experiment. Once your data shows a line then you can calculate the slope of the line. If your data shows a parabola, you can calculate the quadratic equivalent of slope - don't calculate the actual slope and then declare that to be the result.
Sometimes you see people trying to measure the resistance of diodes, or worse, incorporating the resistance of diodes into calculations. What's the voltage across the diode? The current times the resistance, of course...