1965-66, 1992-94, 1994-95, 1998-01: There is no missing main activity time for these years. To fill the missing gaps, codes have been imputed. Note that codes have not been imputed to replace activities in the diaries. The original missing times have been included under imputed personal care (0102), sleep and rest (0104), secondary imputed eating, imputed time away from home (0758), imputed social activity (0971), and imputed travel (1190).

1975-76, 1985: The original files include two main cases which have been coded as missing time: a time gap greater than 10 minutes and the end of diary marker. In addition, a code of 0 was created during data cleaning for episodes which had no reported activity. All these 3 cases are coded as missing activity time.

2003+: All variable codes that could not be constructed for missing time in the original data are coded as item missing. The remaining missing time has been coded under activities such as general personal care, imputed personal or household care, imputed sleep, and imputed time away from home, restaurant, walking, and imputed travel.