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→Three mechanisms (assumptions) of data missing
!rowspan="2"|[Example]<br>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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!style="width:7em"|missing (unobserved) value itself
!style="width:7em"|other observed variable(s)
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!Missing not at random
!MNAR
|style="text-align:center"|<big>YES</big>
|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 missing 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
!MAR
|style="text-align:center"|<big>NO</big>|style="text-align:center"|<big>YES</big>
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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 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
!MCAR
|style="text-align:center"|<big>NO</big>|style="text-align:center"|<big>NO</big>
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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 NOT because of body weights themselves nor 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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