I am familiar with creating linear time trends. For example, if I have study years 1996, 1997, 1998 and 1999; then the linear time trend variable will be coded 1 for 1996, 2 for 1997, 3 for 1998 and 4 for 1999.

However, I am uncertain what to do when I have one year of data missing, for example, if I have data on only study years: 1996, 1998 and 1999 with the 1997 data missing.

Do I code 1998 as 2 or 3 in the linear time trend variable?