Finally, the book also observes that learning happens not only within projects but also across projects (p159). Solar and wind projects are significantly less risky than nuclear projects in part because solar and wind projects deploy hundreds or thousands modular units, rather than one very large unit. Even if some wind turbines are poorly designed or installed, they can learn from than for the next ones. Learning to build nuclear power plants is much harder, since so few of the projects occur.
Composability and volume leads to improvement, as learning can happen during each component implementation.