= Solution
Ask the doctor whether, \b[among patients with the same first-year drug-use status, patients who dropped out would be more or less likely to have used the drug in the second year than those who remained]. The comparison is within the observed first-year categories, not just between all dropouts and all completers.
If there is no remaining relationship with second-year drug use after conditioning on first-year use, <missing at random> is plausible for these recorded variables. If treatment-related difficulties, new medication or deterioration lead to dropout in a way not accounted for by the recorded first-year status, <missing at random> may fail. A different response rate in the two first-year categories is allowed under <missing at random>. The unobserved second-year outcomes mean that the observed table alone cannot establish this assumption; clinical knowledge or additional follow-up information is needed.
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