Normalisation refers to a process scientists use where they measure one thing, and express it as a ratio (“divided by”) of something else to make comparisons easier. It’s really not as complicated as it sounds, and you are almost certainly already familiar with the idea without realising it.
An example from every-day life.
Suppose you want to compare the fuel efficiency of two cars (let’s call them car A and car B). Car A can drive for 400 miles on a full tank of fuel, but car B can only drive 350 miles on a full tank.
This doesn’t really tell us which car is most efficient because the cars may have different size fuel tanks. Maybe car B drove a shorter distance because it has a smaller fuel tank. We can fix this by calculating how far each car went per gallon (or litre) of fuel.
Car A drove 400 miles on a full tank, its tank holds 10 gallons so it did 400 miles/10 gallons = 40 miles per gallon
Car B drove 350 miles on a full tank, its tank holds 7 gallons so it did 350 miles/7 gallons =50 miles per gallon.
Car B is actually more fuel efficient.
We have “normalised” the distance travelled to the fuel capacity of the tank.
Some scientific examples.
Many drugs work by binding to something called a “receptor”, think of it as the thing the drug sticks to for it to have its effect. If we wanted to measure how many receptors there are for a particular drug in different organs of the body (eg kidney, liver, brain), just measuring the total number of receptors wouldn’t make much sense because organs are different sizes. Instead, we could “normalise” the number of receptors to the mass (weight) of the organ and instead state the number of receptors per gram of tissue. That way we could compare the number of receptors in different organs in a meaningful way.
This example is a little more complex. Enzymes are type of protein that make chemical reactions happen more quickly. For example, an enzyme called hexokinase speeds up a reaction in glucose is converted to another chemical called glucose 6 phosphate. We call glucose 6 phosphate the “product” of the reaction. We might want to measure how quickly the product is produced when the enzyme is present. One way we could do this is to measure the amount of product that is produced by the enzyme over 10 minutes. However, scientists in other laboratories might measure the reaction over 5 minutes instead. Not surprisingly, the amount of product formed will be different depending on the period over which we follow the reaction. One solution to this is to express the speed of the reaction as “amount of product formed per minute”, and we calculate this by dividing the amount of product formed by the duration of the reaction, in the same way as we expressed the fuel efficiency of the cars as miles per gallon.
However, the amount of product also depends on how much of the enzyme we add and different scientists may test different amounts. If we add more enzyme the reaction will go even faster and make more product. We can solve this by dividing the amount of product by the amount of enzyme we added. We can measure the amount (mass) of enzyme in milligrams, (1 milligram is 1/1000th of a gram). So we divide the amount of product by the amount of enzyme added, or “amount of product formed, per milligram of enzyme”
Now we need to combine both of those two ideas together so we can take account of the duration of the reaction and the amount of enzyme we used. We end up expressing this as “amount of product formed per minute per mg of protein” by dividing the amount of product formed by the duration of the reaction and also by the mass of enzyme added to the reaction. The advantage of this is that we can now compare different experiments where we might have used different assay duration or a different mass of enzyme.
So next time you are presented with data in expressed as “something per something”, you will know why.