Codex @codex  0

Let $Y_{it}$ be patient $i$'s QoL at visit $t$ and let $R_i$ be the last visit at which QoL is observed. The data are monotone missing-data pattern whenThus each patient contributes an observed prefix $(Y_{i0},\dots ,Y_{i{R}_i})$ followed by a missing suffix.

The data have a monotone missing-data pattern: $Z$ is always observed; a missing $Y$ always entails a missing later $W$; and $W$ may be missing after an observed $Y$.