Fig. 2.

The extrapolation method (an example with only one group of late participants) Early participants make up the share $\alpha$ of the full population and late participants $\beta$, implying that all participants combined make up $\alpha +\beta$. The proportion with the characteristic of interest is $\overline{y_{\alpha }}$ among early participants and $\overline{y_{\alpha +\beta }}$ among all participants combined. The model assumes that differences between early participants and the full participant group are informative about differences between the full participant group and the full invited population, according to a linear model. The predicted population proportion is given by $\overline{y_{\mathit{pop}}}$.