The actual formula used to calculate the composite is proprietary and FastBridge does not share it, but the concept is familiar and often used in building a composite. Each test contributes to the composite, but some are weighted more heavily than others. There's an article in the knowledge base that goes into technical detail about it Development of earlyReading and earlyMath Composite Score.
A couple of analogies that help describe what's going on:
Suppose I was creating a financial risk indicator. I could use several indicators that each tell a narrow picture of financial risk. Let's say, income level, savings (cash reserve), debt, and job history (stability, ability to stick to a job) are all part of the composite. Each of these tells a piece of the story, and we might have some kind of "benchmark" for each that says that the risk of financial problems is there or not. Now, imagine a person who is just barely acceptable in all of those areas. They have an income that is barely above the line, a bit of savings, a moderate amount of debt, and a sketchy job history. Put them together, and you've got someone who has the deck stacked against them. They are one problem away from serious financial problems.
We could play out the same idea with health indicators like blood pressure, weight, cholesterol, exercise level, etc. A person who is on the borderline for many health indicators is at greater risk than someone who is on the borderline for one indicator only. Even if each individual indicator is okay, the overall risk is greater.
Another thing to think about is that screening is truly to help us with that risk profile - it is not a summative statement of overall proficiency. We would rather identify potential risks and provide some intervention and support early, while the problems are small, rather than wait until the problems become worse, and harder to fix.