XED measures the responsiveness of demand to a change in income - If the demand for food remained unchanged if income rose then XED would be 0.
There are 2 ways to calculate XED depending on the information you are given
If you are given numbers you use -
If you are given percentages you use:
Positive and negative numbers
A substitute good will give a positive number. The closer to 0, the weaker the substitute is.
A complementary good will give a negative number. The closer to 0, the weaker the complement is.
If the answer is 0, the two goods in question are said to be unrelated.