Sampling Bias and Nonprobability

Research Bias

MUST ELIMINATE BIAS Only without bias can researchers be confident in their findings Bias occurs when respondents are not selected through true random sampling    

Online Panels - guidelines

invitation preferred over "opt-in" - but it generates a low response rate professional respondents are undesirable verify your respondents are real Panel should represent US population  Monitor Response Rates - low response rates means people are turning away Poor panelists removed (the people who press the same button or answer crazy)  

Nonprobability Sampling

Convenience Judgement Quota Snowball Not good for generalization, only for quick, preliminary results  

Convenience Sampling

respondents are selected because they are convenient and accessible. will most likely provide biased and unreliable information. should only be used when there is no need to generalize to the broader population appropriate for exploratory research or quick information such as questionnaire pretesting times and conditions can change a sample DRAMATICALLY  

Judgement Sampling

respondents selected based on an expert's decision on who is best to interview. the greater the expertise, the greater the confidence in the results of the research (directly related) only recommended when the is immense confidence in the expert opinion or only when preliminary, exploratory information is needed Approaches include: extreme cases typical cases intense or passionate cases  

Quota Sampling

Extension of judgement sampling attempts to represent demographics of the sample equal to that of the broader target population obtained in 5 steps: Determine the defining characteristics of the key subgroups Determine the percent of the total population represented by each defining charateristic Determine the percent of the total population represented by each quota cell. translate the percent into a sample size Sample population  

Snowball Sampling

Uses current participants to recruit new participants used for populations where access is through personal introductions subject to numerous biases can be caused by a reposting or forwarding of surveys by a current participant Can be inaccurate because you have no idea who is answering the survey