CHAPTER 6 :Ways of Expressing Prognosis
person years
Apparent effects on prognosis of improvement in Diagnosis
GENERALIZABILITY OF SURVIVAL DATA
characterize the natural history of disese in quantitative terms
- describe the severity of a disease to establish priorrities for clinical service and public health program
- patien often ask question about prognosis
- quantification is important to establish a baseline for natural history so that as new treatmen become availabe the effect of these treatments can be compare with the expected outocome
is term of the number of deaths divided by the person years over which a group is observed
person years are useful as denominators of rates of event in many situations such as randomized trial & cohort studies
5 APPROACH TO EXPRESSING PROGNOSIS
1 . Case fatality rate
ideally suited to disease : short-term and acute condition
Denominator = number of people who have the disease
is defined as the number of people who die of a disease divided by the number of people who have the disease
- Five Year Survival
Lead time : gap time during early screening time to general time of diagnosis
2 limitation : no temporary change over the period ,lost off follow up, experience with 5 years
Definition : is the percentage of patients who are alive 5 years after treatment begins or 5 years after diagnosis
Application: frequently used in clinical medicine ex: cancer
- OBSERVED SURVIVAL rate
2 APPROACH FOR CALCULATION OBSERVED SURVIVAL OVER TIME
Calculating a Life Table
in 5 years we loss bit chance of data in 5 years , who can follow just 2 or 3 years, so subject who not complete the 5 years still can be come to the calculation data
withdrawals : person for whom data are not available for the full period of follow up either because follow up was not possible or because they were enrolled after the study was stared
Rationale for the Life Table
the use of data is economical and efficient
- MEDIAN SURVIVAL TIME
- RELATIVE SURVIVAL
THE KAPLAN MEIER METHOD
ASSUMTIONS MADE IN USING LIFE TABLE