Data were weighted where necessary by age, gender, race/ethnicity, region, education, marital status, household size, household income, employment, and propensity to be online, to bring them in line with their actual proportions in the population.
Respondents for this survey were selected from among those who have agreed to participate in The Harris Poll surveys. The sampling precision of Harris online polls is measured by using a Bayesian credible interval. For this study, the sample data is accurate to within + 2.8 percentage points using a 95% confidence level. This credible interval will be wider among subsets of the surveyed population of interest.
All sample surveys and polls, whether or not they use probability sampling, are subject to other multiple sources of error which are most often not possible to quantify or estimate, including, but not limited to coverage error, error associated with nonresponse, error associated with question wording and response options, and post-survey weighting and adjustments.
En el primer párrafo indican que utilizaron el producto Harris On Demand. Por lo que he entendido, el producto se basa en poder encuestar al un público objetivo acorde a unos parámetros de selección, siendo los encuestados pagados por completar la encuesta. Como decía, la clave aquí es saber (que no lo sabemos) exactamente cómo fue seleccionado el público objetivo, y si este ya tiene de por sí un sesgo por participar en este tipo de encuestas online de por sí.