A seam is the point at which two separate pieces of material abut. In manufacturing processes, the quality of the product is often dictated by the quality of seams. Longitudinal surveys also have seams, and as with manufactured products, these seams are critical elements in determining the overall quality of the survey. The seams in a longitudinal survey occur in places where data that cover two different interview periods (or waves) are brought together.  The SIPP because it gathers information for four months at a time, at four-month intervals, has many “wave-seams.” The dependency of responses on the change in waves is called the wave-seam effect.