A fold change is basically a ratio. It indicates the number of times something has changed in comparison to an original amount. A twofold increase indicates that an amount doubled. Fold change is useful in examining data for increases and decreases.
Divide the new amount of an item by the original amount to determine the fold change for an increase. For instance, if you have 2 armadillos in a hutch and after breeding, you have 8 armadillos, the calculation is 8/2 = 4. The 4 means that you have a 4-fold increase in the number of armadillos.
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Divide the original amount by the new amount to determine the fold change for a decrease. For instance, if you have 20 grams of water at the beginning of an experiment and end up with 4 grams, divide the original number (20) by the new (4) and note the answer as a negative result. In this case, 20/4 = -5 fold.
Find the fold change in experiment results by dividing the data from the experimental group by the data from the control group. For instance, if you have an experimental group which yields 10 specimens and a control group which only produces 4, the calculation is 10/4 = 2.5 fold.