To find the expected slope for the relationship between mass and the characteristic length of an animal (see figure), the units of mass (M) from the Y-axis are divided by the units of the X-axis, Length (L). The expected slope on a double-logarithmic plot of L3 / L is 3 (formula_6). This is the slope of a straight line, but most data gathered in science do not fall neatly in a straight line, so data transformations are useful. It is also important to keep in mind what is being compared in the data. Comparing a characteristic such as head length to head width might yield different results from comparing head length to body length. That is, different characteristics may scale differently.