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General issues of Vaccine
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General issues of Tropical med.
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General issues of Travel med.
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Trematode (fluke, distoma)
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Three mechanisms (assumptions) of data missing
Mechanisms (assumptions) of data missing
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Abbr.
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Missing depends on/ Missing occurs because of
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[Example] A cohort study in which participants self report their body weight every week via special app installed on their smartphones. Demographic variables are also collected.
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missing value itself
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other observed variable(s)
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Missing not at random
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MNAR
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YES
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—
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- Some participants intentionally denied to report higher body weights they didn't like and only reported lower body weights they accepted
- Researchers have no information why they denied to report body weights in several weeks
- Missing occurred because of the values themselves
- Researchers cannot explain/predict the missing mechanism from other observed (reported) body weights or other variables
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Missing at randam
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MAR
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NO
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YES
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- 15% of female participants denied to report their body weights after participation regardless of measured body weights, but only 1% of male denied to report
- Researchers can identify that female participants more likely denied to report body weights than male
- Missing occurred NOT because of body weights themselves but because of another variable sex
- Missing mechanism is at random, i.e., free from missing values only inside each participant but NOT at random, i.e., NOT free from variable sex
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Missing completely at random
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MCAR
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NO
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NO
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- A participant moved to outside cohort area because of family affair and was lost to follow up
- A participant failed to report body weights because of malfunction of the app
- Missing occurred NOT because of body weights themselves NOR other variables
- Missing mechanism is completely at random, i.e., completely free from both of missing (unobserved) values and observed values in body weight or in other variables
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