5.3 Errors

Sampling Frame * Individuals from the population from which the sample can be obtained from. * Not always the whole population. Example: A political group interested in how people in the state feel about budget cuts takes a sample of 1000 people from registered voter lists. * Sampling frame = registered voter list * Sample = 1000 people * Population = all people in the state (including those not registered to vote)

Errors in Sampling Undercoverage When some groups of the population are left out of the sample On purpose or by accident Examples: Call-in poll (leaves out those w/o phones) Stopping people @ mall (leaves out those who do not go to the mall on that day/time) Mailings (leaves out people w/o mailing addresses, like the homeless) Standing by back door of school in the morning (leaves out those who do not enter the school that way, and also those with late arrival)

Processing Errors Mistakes in collecting or entering the data Examples: - doing arithmetic wrong - typos - recording wrong numbers/info - losing data

Nonresponse Error Failure to obtain data from an individual in the sample Happens because subjects refuse to respond or can’t be contacted Examples: - not answering phone/ hanging up phone - not sending back mailing - absent on day of poll - refuse to write answers on a survey

Voluntary Response Error Error from a voluntary response sample The sample is not representative of the population. Not the same as nonresponse error. For nonresponse, individuals must be selected, and then do not respond. For example, they must be called on the phone, or handed a survey, or received a mailing, or had their number selected in a SRS. For voluntary, you give a general appeal, and individuals select if they want to respond. If they choose not to volunteer, that is fine and is NOT nonresponse error. That’s voluntary response error.

Response Error When subjects give an incorrect response Could be due to the environment, question, or person asking the questions. Examples: - lying (especially with sensitive questions) - remembering info incorrectly - don’t understand question - mislead - influenced by questions/surveyer

Wording of questions * When a question: - is confusing - uses big words or technical language that most people don’t understand - uses a word that has more than one meaning and doesn’t clarify - ARE SLANTED towards one response (based on the question alone or a statement with the question)