Errors in Sampling Objective: to identify the types of nonsampling errors

 Statistics Process  Types of Data  Types of Statistics  Observational Studies vs Experimental Design  Sampling Methods ◦ Simple Random ◦ Stratified ◦ Systematic ◦ Cluster ◦ Convenience

For this reason, we (Harris) include a strong warning in all of the surveys that we publish. Typically, it goes as follows: In theory, with a sample of this size, one can say with 95 percent certainty that the results have a statistical precision of plus or minus __ percentage points of what they would be if the entire adult population had been polled with complete accuracy. Unfortunately, there are several other possible sources of error in all polls or surveys that are probably more serious than theoretical calculations of sampling error. They include refusals to be interviewed (non-response), question wording and question order, interviewer bias, weighting by demographic control data, and screening (e.g., for likely voters). It is difficult or impossible to quantify the errors that may result from these factors.

 Nonsampling error: ◦ Resulting from survey process  Sampling error: ◦ false or incomplete information about a population

 The Frame - possible underrepresentation  Non-response - may be controlled through “Callbacks”  Interviewer Error - essential to obtain “truthful” answers

 Misrepresentative answers - watch for outright lies  Data Checks - check for accuracy at all stages  Loaded Questions - questions should not imply any judgment  Order of Questions - quite often the “vote” goes to the first “candidate” encountered.

 The village of Oaklawn wishes to conduct a study regarding the income level of households within the village. The village manager selects 10 homes in the southwest corner of the village and sends an interviewer to the homes to determine household income.  What is the flaw and what is a possible remedy?

 To determine public opinion of the police department, the police chief obtains a cluster sample of 15 census tracts within his jurisdiction and samples all households in the randomly selected tracts. Uniformed police officers go door to door to conduct the survey.  What is the flaw and what is a possible remedy?

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