Slide 1

How are we doing? Sort the types of error into sampling and non-sampling errors, then match the situations to the types of error. Slide 2 Understanding sampling error and non-sampling error Choosing the correct rule of thumb for margin of error Knowing what to do when proportion is 70% Deciding whether a study is an experiment or an observation Understanding sampling methods Polls and surveys What else? Slide 3 Understanding sampling error and non- sampling error Slide 4 Slide 5 ESA Study guide Level 3 Statistics (and ESA L3 Statistics Learning Workbook) Non-sampling errors result from how information is collected from a sample Examples are: Non-participation False answers Unavailability Lack of opinion No mention of major sources of non-sampling error such as incorrect sampling frame, biased sampling method, survey method, etc. LACK OF CLARITY Slide 6 NO INFORMATION? Slide 7 AME Level 3 Statistics Workbook (revised for 2014) Nulake LACK OF CLARITY Slide 8 but that leaves us with textbooks and exercise books with either no information or unclear information. Where to from here? Slide 9 Sampling error arises due to the variability that occurs by chance because a random sample, rather than an entire population, is surveyed. Non-sampling error is all error that is not sampling error. Slide 10 Unclear or leading questions Sampling frame which doesnt match target population Biased sampling method Interviewer effects Survey format effects Wrong analysis methods Behavioural issues Coding errors Misinterpretation of analysis Transfer of findings Non-response Hidden agenda Slide 11 Choosing the correct rule of thumb for margin of error Slide 12 Slide 13 If the results are within the same survey or poll, then Margin of Error of Difference = 2Margin of Error of the Poll Dependent Probabilities two events from within the same question then these two probabilities are dependent on each other. Independent Probabilities If we have two polls or two independent questions in the same survey that are independent of each other MOE = 1.5(average MOE). Questions can be asked of different groups within the same survey or poll, so difference is not always 2MOE The definition of dependent probabilities makes it seem as though comparing different questions for the same group would be independent. LACK OF CLARITY? Slide 14 Slide 15 ESA Study Guide Level 3 Statistics ESA Study Guide Level 3 Learning Workbook Sigma (not in the book at all) AME Level 3 Statistics Workbook D&D Practice External Assessments others? LACK OF CLARITY? Slide 16 Focus on one group or two groups One group, one answer One group, difference Two groups, difference Improve our resources Slide 17 Slide 18 Slide 19 comparing answers to the same question (eg National or Labour) for one group is different from comparing the answers to two different questions for one group. Slide 20 Slide 21 Slide 22 Slide 23 Slide 24 Slide 25 It is enough for students to know that there are formulas that simplify to the rules of thumb for percentages between 30% and 70%. Students should not be memorising complicated formulas to use below 30% or over 70%. Students should not be memorising one full formula and substituting it into the other rules of thumb. Slide 26 Knowing what to do when proportion is 70% Slide 27 Explain why it would be inappropriate to use the reported margin of error to construct a confidence interval for the percentage of respondents from the November 2009 survey who never talk to their friends on a landline. (the reported percentage was 9%) Slide 28 Confidence interval using the rule of thumb Interpret CI: With at least 95% confidence, we can infer that the percentage of adult New Zealanders with health insurance who will have had their blood pressure checked during the previous 12 months is somewhere between 87.4% and 92.6%. Slide 29 that they can be more confident that the true population proportion (or percentage point difference) is within the rule of thumb based interval if the proportion(s) is/are below 30% or over 70%. At least 95% confidence conveys this understanding. the actual level of confidence for the rule of thumb confidence interval is higher than 95%. the true 95% confidence interval would be narrower than the rule of thumb confidence interval. margin of error describes half of a 95% CI so we cant call the result of our rule of thumb calculation a MOE unless 0.3