# Two-phase sampling notation

Attention conservation notice: an attempt to get a small number of other people, probably not including you, to adopt our notation.

Together with groups at the University of Pennsylvania and Vanderbilt, we have been working on methods for the design and analysis of two-phase samples, samples taken from an existing cohort or database to measure new variables. The problem combines measurement-error, missing-data, and sampling ideas, so questions of notation can get fraught. For example, there are otherwise reasonable people who would like $$W$$ to be something other than a vector of weights.

Here is an attempt at notation

• We sample $$n$$ observations from a cohort of $$N$$, where the $$i$$th observation is sampled with known probability $$\pi_i$$. Often the sampling is independent (or independent except for fixed $$n$$); if not, we also know the pairwise probability $$\pi_{ij}$$ that both $$i$$ and $$j$$ were sampled.
• The sampling weights $$w_i$$ are $$1/\pi_i$$ or adjusted versions of this to incorporate cohort-level information
• We have variables $$Z$$, $$A$$, and (typically) $$Y$$ measured for everyone in the cohort and $$X$$ measured on the subsample.
• $$R_i$$ is the indicator that observation $$i$$ is in the subsample, so $$E[R_i|Z,A,Y]=\pi_i$$.
• The outcome model is for $$Y|Z,X$$. It is the model we would fit if we had complete data. Its parameters are $$\beta$$; its loglikelihood is $$\ell_i(\beta)$$; its score function is $$U_i(\beta)$$
• The imputation model is for $$X|Z,Y,A$$. Its parameters are $$\alpha$$. It may be used to produce single imputations $$\hat X_i$$ or multiple imputations $$\hat X_i^{(m)}$$ or $$X^*_{i(m)}$$ for $$m=1,2,\dots,M$$.
• The phase-1 model is for $$Y|\hat X, Z$$. It has influence functions $$h_i(\beta)$$. Or for multiple imputation it is for $$Y|X^*_{i(m)}, Z$$ and has influence functions $$h_{i,m}(\beta)$$
• We use the term raking (or generalised raking) for the adjusted-weight estimators, to avoid confusion with the unrelated ‘regression calibration’ technique in the measurement-error literature. But we still call the equations $\sum_{i\in\textrm{sample}}\frac{g_i}{\pi_i}h_i\equiv\sum_i w_iR_ih_i=\sum_i h_i$ that constrain the adjusted weights $$w_i=g_i/\pi_i$$ ‘the calibration constraints’.
• On occasion, we may use $$Y^*$$ and $$X^*$$ for elements of $$A$$ that are versions of $$Y$$ and $$X$$ measured with error, because tradition. Obviously we won’t use the stars to indicate multiple imputation when we do.

The literature has not really made a consistent choice between $$X$$ and $$Z$$, though there is a tendency in measurement-error papers for $$X$$ to be the true predictor value, which fits our notation. The distinction between $$Z$$ and $$A$$ is that $$Z$$ would be in the outcome model even if you had $$X$$ for everyone, and $$A$$ would not. In a classical measurement error approach, the mismeasured covariate would uninteresting if you had the true value, so it would be an $$A$$, not a $$Z$$.

When $$Y$$ isn’t measured on everyone (eg, $$Y$$ is measured with error on the whole cohort and accurately on the subsample), the imputation model doesn’t have $$Y$$ on the RHS.