When targeting an audience representing the US Internet population, Consumer Surveys attempts to find respondents that match the distribution of people in the US by age, gender, and location as reported in the US Census Current Population Survey (CPS). The sampling-bias table at the bottom of the survey’s results page tells you the difference between the collected-answer distribution and the desired distribution from CPS. At the bottom of this table is a root-mean-square error (RMSE).
The RMSE measures the differences between the desired distribution and actual distribution for each targeted population segment and calculates a weighted average error. The RMSE technique weights large errors more than small errors. Thus, if the difference in one segment is very large, it would have a greater effect on the RMSE than if there were small errors across several segments. The lower the RMSE score, the closer we are to representing the US Internet population.