Increasing the sample size tends to reduce the sampling error; that is, it makes the sample statistic less variable. However, increasing sample size does not affect survey bias. A large sample size cannot correct for the methodological problems (undercoverage, nonresponse bias, etc.) that produce survey bias. (stattrek.com)

If, for example, the purpose is measuring average household income and the households without a phone have lower average income than others, ignoring the nonphone group will lead to an upward bias of average household income in the population. Of course the size of the bias would probably be limited in this example, because the group of households without a phone connection is small. (rand.org)

The Internet is an attractive mode of data collection to survey researchers due to cost savings and timeliness in comparison with other modes. However, survey estimates are subject to coverage bias if sampled persons with Internet access are systematically different from those without Internet access who were excluded from the survey. (European Survey Research Association)