Table 1.
Conditions for identifying causal quantities for generalizability and transportability.
| Key identifiability conditions | Meaning | Example illustrations using the NLST |
|---|---|---|
| (i) Conditional exchangeability over study participation, S (S-admissibility) | The participants enrolled in the trial sample are exchangeable with individuals in the target population conditional on some pre-intervention or background variables; i.e., the mean potential outcome conditional on these variables is independent of RCT participation. | The participants enrolled in the trial sample of our example illustration (23 centers with females ≥40%) are exchangeable with those in the target population (10 centers with females <40%) or with the entire NLST conditional on sex, age, race/ethnicity, education, marital status, smoking, body mass index, and the recruitment arm of NLST (LSS or ACRIN). |
| (ii) Conditional exchangeability over intervention in the study participant population | The participants in the intervention group are exchangeable with the participants in the control group conditional on some pre-intervention or background variables. | The participants in the LDCT screening group are exchangeable with the participants in the radiography group, which is expected to hold by randomization in the NLST. |
| (iii) Positivity of RCT participation and intervention assignment | a) There is a non-zero probability of trial participation in any stratum defined by covariates that are needed to ensure conditional exchangeability.a | a) The probability of being in the trial sample is not zero in any stratum defined by age, sex, race/ethnicity, education, marital status, smoking, body mass index, and the recruitment arm of NLST. |
| b) There is a non-zero probability of intervention assignment in any stratum defined by covariates that are needed to ensure conditional exchangeability.a | b) The probability of being in the LDCT screening group is not zero, which is expected to hold by randomization of intervention assignment in the NLST. | |
| (iv) Consistency | The potential outcome under a specified intervention for any individual who received that intervention is equal to the individual's observed outcome. | The potential outcome under LDCT screening for any individual who received that screening is equal to the individual's observed outcome. |
| (v) No interference | One individual's intervention does not affect another individual's outcome.b | One participant's lung screening using LDCT does not influence other participants' lung cancer mortality. |
| (vi) No measurement error | Each variable used in the analyses is correctly measured. | All variables in the NLST are correctly measured. |
| (vii) Correct model specification | The models used in the analyses are statistically correctly specified. | The logistic regression model used to determine whether the participant is in the trial sample and the Cox proportional hazard model used to predict lung cancer mortality were correctly specified. |
Abbreviations: ACRIN, American College of Radiology Imaging Network; LSS, Lung Screening Study.
aFor transportability, the probability is considered for the superpopulation that gave rise to the trial sample.
bOr the pattern of interference is the same between the trial sample and the target population.