Potential non-response bias may increase as there are expected increases in cell-phone only and broadband telephone households. This may be particularly problematic if use of these types of alternative phones is associated with race, education and poverty which are also factors associated with immunization coverage. Another weakness of the NIS is the inability of the NIS to measure coverage within IAP areas due to the cost prohibitive nature of within state sampling. As discussed earlier, measuring coverage on the local level is very important.