After design, the next critical step is selection of specific components and vendors. In addition to meeting the specifications called out by the designer, these components must be compatible with the manufacturing process. Magnetic components, although conceptually simple, require extra attention as well. If they are not properly designed or assembled, or if their core halves are not glued properly, they can begin to vibrate at audio frequencies. Vendor credibility and conformance is key. The reason is that it's difficult to distinguish a properly and consistently manufactured component such as a capacitor from a poorly made unit. Finally, failure is significantly more likely during the early stages of a component’s life than during its useful life. Burn-in testing weeds out units that would have failed early in the field and therefore would have brought down the overall reliability rating.