As mentioned above, verification is one of the most integral parts of the design control process and probably where engineers encounter the most problems. This is the first step where you really get to see if your prototype is on the right track towards becoming a successful product according to your customer needs. These customer needs are the inputs for the product and define all of the functional requirements which need to be included. If you do not meet your customer needs, then essentially there is no customer and no market to sell your product. Each input requirement will have a corresponding output feature which was included in the design to fulfill that need. Various test methods need to be employed to do this verification check and determine whether or not some changes need to be made to the design. I think one problem with verification is the leniency in which a company may accept a certain output as satisfying its original requirement. If you want to check the color of a part you could do a visual inspection and determine it is red. You could also use a spectrophotometer and obtain the exact color values. Both test methods will determine the part is red and thus a successful design feature but one is inherently more accurate in determining compliance. So engineers need to decide which tests are qualified to determine success.