Survival-time (st) datasets like aids.dta contain information on individual people or things, with variables indicating the time at which failure or censoring occurred for each individual. A different type of dataset called count-time (ct) contains aggregate data, with variables counting the number of individuals that failed or were censored at time t. For example, diskdriv.dta contains hypothetical test information on 25 disk drives. All but 5 drives failed before testing ended at 1,200 hours.
To set up a count-time dataset, we specify the time variable, the number-of-failures variable, and the number-censored variable, in that order. After ctset, the cttost command automatically converts our count-time data to survival-time format.
The cttost command defines a set of frequency weights, w, in the resulting st-format dataset. st* commands automatically recognize and use these weights in any survival-time analysis, so the data now are viewed as containing 25 observations (25 disk drives) instead of the previous 6 (six time periods).
Source: Hamilton Lawrence C. (2012), Statistics with STATA: Version 12, Cengage Learning; 8th edition.
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