As much as you try to predict what styles will be the most popular, you aren’t always right. Each style and collection is an experiment.
Through research on prior collections, intuition, and experience, you make a design hypothesis. You have a guess at what will sell, but it isn’t proven yet.
In order to know for sure if the guess is right, you have to put the design out there and test it. Decide what criteria will make it a success or failure and then take a step forward based on the data you have available. See the reaction. Record your findings.
Some styles might fail, but at least you learned what didn’t work and have a better idea of what to try next.
Sometimes, though, your hypothesis is spot on and that design becomes the basis for future pieces. You’ll never know which it is until you try it.