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→Three mechanisms (assumptions) of data missing
!style="witdh:4em" rowspan="2"|Abbr.
!colspan="2"|Missing depends on/<br>Missing occurs because of
!rowspan="2"|[Example]<br>A cohort study in which participants self report their body weight every week via special app installed on their smartphones for 52 weeks in total. Demographic variables are also collected.
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!style="width:7em"|missing value itself
|style="text-align:center"|—
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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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|style="text-align:center"|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, but it's applicable only inside each participant**Missing mechanism is but NOT at random, i.e., NOT free from variable ''sex''; missing occurred depending on the values in the variable ''sex''
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!Missing completely at random
|style="text-align:center"|NO
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*A participant '''moved to outside cohort area ''' because of family affair and was lost to follow up after the 10th week*A participant '''failed to report body weights because of malfunction of the app between the 15th and 17th weeks'''
**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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